It was initially implemented in #5871 for Godot 3.0, but never really
completed or thoroughly tested for most platforms. It then stayed in
limbo and nobody seems really keen to finish it, so it's better to
remove it in 4.0, and re-add eventually (possibly with a different API)
if there's demand and an implementation confirmed working on all
platforms.
Closes#8770.
Also drop some unused files.
Renamed:
- `platform/iphone/sem_iphone.h` -> `semaphore_iphone.h`
(same for `osx`)
- `platform/uwp/gl_context_egl.h` -> `context_egl_uwp.h`
- in `platform/windows`: `context_gl_win.h`, `crash_handler_win.h`,
`godot_win.cpp`, `joypad.h` and `key_mapping_win.h` all renamed to
use `windows`. Some classes renamed accordingly too.
- `EditorExportAndroid` and `EditorExportUWP` renamed to
`EditorExportPlatformAndroid` and `EditorExportPlatformUWP`
- `power_android` and `power_osx` renamed to `PowerAndroid` and
`PowerOSX`
- `OSUWP` renamed to `OS_UWP`
Dropped:
- `platform/windows/ctxgl_procaddr.h`
