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)

Conflicts:
	src/library_sdl.js
v1.18.2
Merge pull request #2362 from juj/sdl_windowevent_focus

Sdl windowevent focus