Implement the hardware keyboard interface

[swift-kim]

Closes #221.

Please read this page for differences between the legacy RawKeyboard API and the new HardwareKeyboard API.

Since the legacy API has not been deprecated yet, a key event must be sent to the framework twice through the conventional platform channel and also the new embedder API (FlutterEngineSendKeyEvent) simultaneously. In our code, it's done by the SendChannelEvent and SendEmbedderEvent methods in the KeyEventChannel class respectively. As commented in the code, the KeyEventChannel class will be renamed and refactored once the old API is fully removed from the framework.

For the implementation, I referred to the desktop embedders (Windows and Linux) code. Since there's no numeric value that represents a "logical key code" on Tizen, I created a direct mapping (kSymbolToLogicalKeyCode) from key symbols (string) to Flutter's logical key codes.

There are also some other changes to code related to the legacy API:

• Move the GTK key mapping to the key_mapping.cc file.
• Remove symbol-to-scan code conversion (kSymbolToScanCode). This will affect existing apps that depend on physical key codes such as mediaPlayPause (the Play/Pause button on TV) and contextMenu (the Menu button on mobile).
• Get a UTF32 code point from string instead of compose.
• XF86SysMenu (TV remote control's Menu button which opens up the TV settings) has been removed from kBindableSystemKeys and now it's never handled by the app.

I'll do some more testing in this week. To test this change, you may create an app that contains Focus widgets such as KeyboardListener (new API) and RawKeyboardListener (legacy API) and implement onKey or onKeyEvent.

[swift-kim]

@bbrto21 The error seems to be fixed by dcd66c1. Could you please check again?

[bbrto21]

@swift-kim Thanks, problem solved!

[JSUYA]

LGTM :)