Move GTK-specific files to Source/Platform/gtk

GTK-specific source files are moved to Source/Platform/gtk.
Forwarding header generation script in WebKit2 and various includes
of the headers being moved or paths pointing to build targets are
adjusted. The no-op directories are removed from the CPPFLAGS lists.
Generate UserAgentGtk.h into DerivedSources/Platform

Place the UserAgentGtk.h header into DerivedSources/Platform.
Inclusion callsites are adjusted, platform library CPPFLAGS enhanced
and built platform sources added to the main list of built sources.