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.
File introduced in rP9, became this include-only file in rP28.