Refactor PlatformChannel

[swift-kim]

Before:

• platform_channel.cc & platform_channel_stub.cc

After:

• platform_channel.cc: Common code that processes arguments and sends results.
• platform_channel_tizen.cc: Tizen-specific implementation.
• platform_channel_linux.cc: Linux desktop implementation.

Notable changes:

• JSON (method arguments) processing is always performed by the PlatformChannel class. Thus, clipboard::GetData and clipboard::SetData are no longer required.
• FeedbackManager has been refactored for simplicity and readability.
• SystemSound and HapticFeedback no longer throw exceptions. Errors are logged to the console via FT_LOG(Error).

[swift-kim]

The test code I used for testing:

Column(
  mainAxisAlignment: MainAxisAlignment.center,
  children: <Widget>[
    ElevatedButton(
      onPressed: () async {
        var data = await Clipboard.getData(Clipboard.kTextPlain);
        print(data?.text ?? 'null');
      },
      child: Text('Clipboard.getData'),
    ),
    ElevatedButton(
      onPressed: () async {
        await Clipboard.setData(
            ClipboardData(text: 'Counter: $_counter'));
        _counter++;
      },
      child: Text('Clipboard.setData'),
    ),
    ElevatedButton(
      onPressed: () async {
        await SystemSound.play(SystemSoundType.alert);
      },
      child: Text('SystemSound.play (alert)'),
    ),
    ElevatedButton(
      onPressed: () async {
        await SystemSound.play(SystemSoundType.click);
      },
      child: Text('SystemSound.play (click)'),
    ),
    ElevatedButton(
      onPressed: () async {
        await HapticFeedback.vibrate();
        print('brrr!');
      },
      child: Text('HapticFeedback.vibrate'),
    ),
    ElevatedButton(
      onPressed: () async {
        await SystemNavigator.pop();
      },
      child: Text('SystemNavigator.pop'),
    ),
    ElevatedButton(
      onPressed: () async {
        _landscape = !_landscape;
        await SystemChrome.setPreferredOrientations(<DeviceOrientation>[
          if (_landscape) DeviceOrientation.landscapeLeft,
        ]);
        print('landscape = $_landscape');
      },
      child: Text('SystemChrome.setPreferredOrientations'),
    ),
  ],
),

[bbrto21]

LGTM

[bwikbs]

Goooood!! 👍