Introduce our own shutdown callback mechanism.

Instead of adding stuff to `_shutdown()`, we can now register a shutdown
routine from the respective `init()` function of our modules.

This also has the advantage, that we're sure they're called in reverse
order from how the initialization happened.

I didn't simply use `atexit()` because POSIX says that the number of
possible callbacks is limited (min 32) and I was not sure whether
we will maybe extend this number at one point.

Signed-off-by: Steffen Jaeckel <s@jaeckel.eu>

src/ui/ui.h

@@ -62,7 +62,6 @@
 void ui_init(void);
 void ui_load_colours(void);
 void ui_update(void);
-void ui_close(void);
 void ui_redraw(void);
 void ui_resize(void);
 void ui_focus_win(ProfWin* window);