Almost all of the files in source/gui/ match a classname that is implemented.
This file makes an important impression as it is named like the root folder GUI, yet it includes nothing really useful,
because many of the includes are not used by most files.
Additionally including "GUI.h" instead of gui/GUI.h is against the coding convention order of includes.
The file is also easily to confuse with CGUI.h which relates to an actually implemented important class.
Supersede typedef with using while at it.