MultiLineTextInput Disable/enable is not working in Cocoa

[ignaciocabeza]

Expected Behavior

It should be possible disable/enable MultiLineTextInput in Cocoa.

Current Behavior

Enable/Disable feature in MultiLineTextInput is not working. (ref: https://github.com/beeware/toga/blob/master/examples/multilinetextinput/multilinetextinput/app.py#L9)

Additional Info

In this widget self.native is a NSScrollView. set_enabled is updating self.native.enabled but text widget lives in self.text
(https://github.com/beeware/toga/blob/master/src/cocoa/toga_cocoa/widgets/multilinetextinput.py#L20-L30)

Also, self.text does not have enabled property. I found only editable and selectable. Maybe a enabled property should set this two properties?. The only this that I can't reproduce is show a "gray disabled apparence" when enabled == false
Example code:

    def set_enabled(self, enabled):
        self.interface.readonly = not enabled
        self.text.selectable = enabled

Steps to reproduce

Run https://github.com/beeware/toga/blob/master/examples/multilinetextinput/multilinetextinput/app.py and press toggle enabled

Your Environment

• Python Version (list the specific version number)
  3.7

• Operating System and Version (select from the following and list the specific version number; if your OS is not listed, list that as well)

  • macOS - version:
  • Linux - distro: - version:
  • Windows - version:
  • Other - name: - version:

• Toga Version (list the specific version number or git hash)
  0.3.0.dev20

• Toga Target (the type of app you are trying to generate)

  • android
  • cocoa
  • django
  • gtk
  • iOS
  • tvOS
  • watchOS
  • winforms
  • win32
  • Other (please specify)

[freakboy3742]

Fixed by #1938.