Don't pause execution if the window loses focus during menus or replays

[nmlgc]

People might want to leave a replay or the Music Room running while the game is not in focus. Active gameplay should probably always be paused when the window loses focus though, and pop up the pause menu when it regains focus, just like modern Touhou.
DxWnd offers this functionality for the old graphics backend via the Main → Generic → Do not notify on task switch option, but it would need to be a proper part of the game when using the SDL backend implemented in #4.

In any case, this should be a config option.