Fixed, cleaned up and standardised header inclusions.
Apart from the clean up and the readability improvements, this fixes a bunch of broken private header inclusions (#include "foo_p.h" in private headers instead of "private/foo_p.h" or better <Project/private/foo_p.h>), now all the private headers of the UITK can be included by the users withour compilation issues. It also removes a bunch of direct includes from sources and a few INCLUDEPATHs hacks.