GTK3: Replace GDK_<keyname> with GDK_KEY_<keyname>

Key constants have gained a _KEY_ infix. For example, GDK_a is now GDK_KEY_a. In GTK+ 2, the old names continue to be available. In GTK+ 3 however, the old names will require an explicit include of the gdkkeysyms-compat.h header.

See https://developer.gnome.org/gtk3/3.9/gtk-migrating-2-to-3.html#id-1.6.3.3.6

Dirty hack was added to src/CMakeLists.txt in order to disable gdkkeysyms-compat.h header and make old names unusable to prevent developers from using them.