Update Visual Studio Projects

* Purge some of the unecessary entries form the project files, notably the
  empty items in the Visual Studio 2005/2008 projects and linking to
  gthread-2.0.lib, as the gthread library is considered deprecated and it
  is no longer necessary to link to it since quite some time ago.
* Clean up the formatting of the Project Files.
* Fix the include paths on including the libsigc++ headers, notably
  sigc++config.h.
* Update the glibmm and giomm projects as newer sources were added so that
  these libraries will build properly.