New plugin supporting the Window API

[JSUYA]

We may need to provide a Dart API for controlling Windows.
(We are already providing a way to control the window using the internal method channel. flutter-tizen/embedder#122)

However, some plugins already obtain the window native handle, open to libecore_wl2.so using dlopen, and make EcoreWl2WindowProxy.
https://github.com/flutter-tizen/plugins/blob/master/packages/video_player_avplay/tizen/src/ecore_wl2_window_proxy.h

Therefore, we can provide APIs as plugins for several APIs.

ecore_wl2_window_show
ecore_wl2_window_hide

ecore_wl2_window_activate
ecore_wl2_window_raise
ecore_wl2_window_lower

ecore_wl2_window_alpha_get
ecore_wl2_window_alpha_set

ecore_wl2_window_title_set   // ??
ecore_wl2_window_title_get  //??

If you have any opinions, please feel free.

[xiaowei-guan]

@JSUYA Do you need me to implement it?

[JSUYA]

@JSUYA Do you need me to implement it?

No, thank you. I'll do it when I have time later. It's not urgent.