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.