Made pick_font_size() pick a more realistic size when the X dimension is

Made pick_font_size() pick a more realistic size when the X dimension is
smallest by multiplying the X dimension by 1.2.

Greatly simplified paint_button() and added four extra arguments to it:
xp, yp, w, and h. Used this new function to blit six buttons to the main
menu. Created two functions to detect whether the mouse is in a button
and to handle things if it is. The data should be stored in a static
variable rather than get duplicated.

Made SDL_QUIT, resize_menu_images(), etc get handled by menu_event() etc
instead of loop.c::process_event().

Changed the FPS message to be more legible and to fit onto the screen
more often.

Removed some header files that were not being used.

All of these changes have been tested with Valgrind.