Adds support for SDL Window Events with event ids:
- `SDL_WINDOWEVENT_FOCUS_GAINED`
- `SDL_WINDOWEVENT_FOCUS_LOST`
- `SDL_WINDOWEVENT_SHOWN`
- `SDL_WINDOWEVENT_HIDDEN`

Includes a fix for FFOS phones that prevent the `blur` event when
audio is playing (Bugzilla 910340)
Merge branch 'sdl-visibility' into new-beta2

* sdl-visibility:
  Adds support for SDL Window Events with event ids: - `SDL_WINDOWEVENT_FOCUS_GAINED` - `SDL_WINDOWEVENT_FOCUS_LOST` - `SDL_WINDOWEVENT_SHOWN` - `SDL_WINDOWEVENT_HIDDEN`

Conflicts:
	src/library_sdl.js