Add icon/text allow overlap configuration

[tobrun]

Closes ##119

This PR adds configuration for enabling/disabling:

• SymbolManager#iconAllowOverlap
• SymbolManager#iconIgnorePlacement
• SymbolManager#textAllowOverlap
• SymbolManager#textIgnorePlacement

These configurations apply to all symbols maitained by the given manager.

[m0nac0]

Looks great to me, I was just wondering if we could include some documentation, like a link to this page to explain what these properties/methods do?

[TimothySealy]

I've implemented the configuration for the symbol manager on iOS. The SymbolManager#textIgnorePlacement now throws a FlutterMethodNotImplemented because the Annotation Extension package does not seem to expose this option. Opened an issue here: mapbox/mapbox-annotation-extension#66

[m0nac0]

@tobrun @TimothySealy I've just added the documentation with the link and resolved the simple merge conflict, is this ready to be merged?

[TimothySealy]

@m0nac0 I think this PR is ready to be merged. The missing iOS textIgnorePlacement implementation is tracked in issue #293

[fotiDim]

I am getting compilation errors after this got merged:

Compiler message:
../lib/src/controller.dart:376:11: Error: The getter '_channel' isn't defined for the class 'MapboxMapController'.
 - 'MapboxMapController' is from 'package:mapbox_gl/mapbox_gl.dart' ('../lib/mapbox_gl.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named '_channel'.
    await _channel.invokeMethod('symbolManager#iconAllowOverlap', <String, dynamic>{
          ^^^^^^^^                                                      
../lib/src/controller.dart:383:11: Error: The getter '_channel' isn't defined for the class 'MapboxMapController'.
 - 'MapboxMapController' is from 'package:mapbox_gl/mapbox_gl.dart' ('../lib/mapbox_gl.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named '_channel'.
    await _channel.invokeMethod('symbolManager#iconIgnorePlacement', <String, dynamic>{
          ^^^^^^^^                                                      
../lib/src/controller.dart:390:11: Error: The getter '_channel' isn't defined for the class 'MapboxMapController'.
 - 'MapboxMapController' is from 'package:mapbox_gl/mapbox_gl.dart' ('../lib/mapbox_gl.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named '_channel'.
    await _channel.invokeMethod('symbolManager#textAllowOverlap', <String, dynamic>{
          ^^^^^^^^                                                      
../lib/src/controller.dart:397:11: Error: The getter '_channel' isn't defined for the class 'MapboxMapController'.
 - 'MapboxMapController' is from 'package:mapbox_gl/mapbox_gl.dart' ('../lib/mapbox_gl.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named '_channel'.
    await _channel.invokeMethod('symbolManager#textIgnorePlacement', <String, dynamic>{
          ^^^^^^^^                                                      
                                                                        
Compiler message:                                                       
../lib/src/controller.dart:376:11: Error: The getter '_channel' isn't defined for the class 'MapboxMapController'.
 - 'MapboxMapController' is from 'package:mapbox_gl/mapbox_gl.dart' ('../lib/mapbox_gl.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named '_channel'.
    await _channel.invokeMethod('symbolManager#iconAllowOverlap', <String, dynamic>{
          ^^^^^^^^                                                      
../lib/src/controller.dart:383:11: Error: The getter '_channel' isn't defined for the class 'MapboxMapController'.
 - 'MapboxMapController' is from 'package:mapbox_gl/mapbox_gl.dart' ('../lib/mapbox_gl.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named '_channel'.
    await _channel.invokeMethod('symbolManager#iconIgnorePlacement', <String, dynamic>{
          ^^^^^^^^                                                      
../lib/src/controller.dart:390:11: Error: The getter '_channel' isn't defined for the class 'MapboxMapController'.
 - 'MapboxMapController' is from 'package:mapbox_gl/mapbox_gl.dart' ('../lib/mapbox_gl.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named '_channel'.
    await _channel.invokeMethod('symbolManager#textAllowOverlap', <String, dynamic>{
          ^^^^^^^^                                                      
../lib/src/controller.dart:397:11: Error: The getter '_channel' isn't defined for the class 'MapboxMapController'.
 - 'MapboxMapController' is from 'package:mapbox_gl/mapbox_gl.dart' ('../lib/mapbox_gl.dart').
Try correcting the name to the name of an existing getter, or defining a getter or field named '_channel'.
    await _channel.invokeMethod('symbolManager#textIgnorePlacement', <String, dynamic>{
          ^^^^^^^^                                                      

[andrea689]

@fotiDim i fix this on #303