Unified handling of SDL_MOUSEBUTTONUP, SDL_MOUSEBUTTONDOWN, and SDL_MOUSEMOTION in...

Unified handling of SDL_MOUSEBUTTONUP, SDL_MOUSEBUTTONDOWN, and SDL_MOUSEMOTION in game.c, menu.c, and options.c, putting the result in loop.c. (The old code is still there, just commented out.) This was quite straightforward: some functions had to be made non-static; data had to be changed to game_data etc; and if statements had to be placed everywhere a file-specific function was called.

Made set_*_widget_sel() and *_perform_click() (of game.c, menu.c, and options.c) non-static so that they could be called from loop.c.

Changed process_event()'s SDL_ACTIVEEVENT to use event rather than calling SDL_ActiveState(), the latter being a vestige of the application-focus-bug debugging.

Moved the GDB variable out of src/Make* and into ./Make*, where it is actually used; and removed depend from the list of .PHONY targets, since it's a real file now.

Ran make depend to create the contents of the file depend.

Added to and changed TODO slightly.