Add default handler for Alt+Tab while keyboard grab is enabled

By default, we will minimize the window when we receive Alt+Tab with a
full-screen keyboard grabbed window to allow the user to escape the
full-screen application.

Some applications like remote desktop clients may want to handle Alt+Tab
themselves, so provide an opt-out via SDL_HINT_ALLOW_ALT_TAB_WHILE_GRABBED=0.
2 files changed