Zoom level global handling

[emdete]

please consider to put the zoom level limitation into the library. currently there are 3 places where zoom levels can be set:

• at the online tile class
• at the tile layer
• at the zoom controls

none of these talk to each other automatically (for example if a online tile may have smaller limits than set in the controls).

[devemux86]

For reference we have modelled the MapViewPosition core class which handles centrally the map position and its limits.

The related discussion in the forum is here.

[devemux86]

Related to #662.

[devemux86]

I added setZoomLevelMin and setZoomLevelMax methods to MapView, handling also the zoom controls.