after an app is minimized, the app icon does not respond when you click
the app icon on the Desktop

we recognized that add lock will lose more time in submit_layers,
and that lock is unuseful. Remove this lock and use origin logic.

1.rename "register_event" -> "user_window_event",
  rename "get_register_event()" -> "get_user_window_event()".
2.add function user_event_function() to reduce Cyclomatic complexity of process_event
  in file platform/sdl/platform.cpp.
3.add Log when SDL_RegisterEvent return -1.
4.add lock in layer_composer.cpp:submit_layers to protect window in win_layers.

1. adjust code.
2. delete useless annotation

Remove tab and add space instead in window module.

x11 impliments(X11_XSetICFocus, X11_XUnsetICFocus, X11_XDestroyIC, X11_XCreateIC) are "thread safe".
In anbox, they are called by SDL_WaitForEventTimeout, SDL_CreateWindow, SDL_DestroyWindow.
But origin anbox code sets them in different thread, and then crashed in a low probability.
This commit sets them in the same thread to avoid this crash.

This commit is for android system to add gps features.

Remove black window which appear after user click the closing window
button in the host sometimes.

For debugging and development purposes it's sometimes useful to run the
full Android system in a single and statically sized window. This is not
going to be a widely used mode but can be helpful at times.

Also makes our window/layer management a bit more modular to allow easy
additions of new strategies/impementations.

This helps us to collect information about an app (title, ..) we can
reuse for certain other things like window construction.

Audio playback still has a delay about 1-2 seconds but it's usable.

This also moves the platform policy class into its own namespace to be
not specific to just window management.

Mostly based on the protobuf based implementation from Mir.