Refactor example & update readme

[miDeb]

Changes I made:
Example:

• removed _setTargetPlatformForDesktop, as it is no longer required.
• use Theme.of(context) instead of Provider.of<ThemeModel>(context), because that returns the actual ThemeData object, is probably already familiar to users, and does not require knowledge (and an import) of provider.

Readme:

• Updated the example in the readme with the actual example, because that's where users will probably copy-paste from.
• Removed a reference to scoped_model, and added a short info about provider.
• Fixed the screenshots.

Sorry for not splitting this into smaller pieces :)