Cleanup includes

Improved manually with the help of neovim LSP warnings and iwyu:

    iwyu -Ibuilddir/app/ -Iapp/src/ app/src/XXX.c

app/src/shortcut_mod.h

@@ -3,6 +3,7 @@
 
 #include "common.h"
 
+#include <assert.h>
 #include <stdbool.h>
 #include <stdint.h>
 #include <SDL2/SDL_keycode.h>