Visual Studio builds: Generate .pc files

Visual Studio builds: Generate .pc files

Generate .pc files for the package, so that it would be easier for
building introspection for packages that depend on GTK+.  Also split
PythonPath into PythonPath and PythonPathX64 to facilitate the build of
introspection files, which need to have Python that is built with the
same ac=rchitecture where GTK+ is built.

Clean up the formatting and spacing a bit.