Expose separate keyboard and mouse grab support

This adds SDL_SetWindowKeyboardGrab(), SDL_GetWindowKeyboardGrab(),
SDL_SetWindowMouseGrab(), SDL_GetWindowMouseGrab(), and new
SDL_WINDOW_KEYBOARD_GRABBED flag. It also updates the test harness to exercise
this functionality and makes a minor fix to X11 that I missed in

To fit in with this new support, SDL_WINDOW_INPUT_CAPTURE has been renamed to
SDL_WINDOW_MOUSE_CAPTURE with the old name remaining as an alias for backwards
compatibility with older code.
12 files changed