Build libWebCoreGtk.la as libPlatform.la

The build rules for the libPlatform.la static library are now defined
in Source/Platform/GNUmakefile.am. Sources are still located under
Source/WebCore, though.
Roll libWebCorePlatform into libPlatform

Build libWebCorePlatform sources as a part of libPlatform. This again
brings all the compiled source code under Source/WebCore/platform
(and some other platform-specific code the GTK port uses) into one
library. This will ease the process of moving all these source files
from the Source/WebCore directory to the Source/Platform directory,
starting with the GTK-specific code and someday finishing by bringing
over all of the platform layer code.
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.