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.
* Clean up the formatting of the Project Files.
* Fix the include paths on including the libsigc++, glibmm, pangomm and
  atkmm headers, notably the *config.h's of these dependencies.
* Update the gtkmm and gtk-demo projects as newer sources were added so
  that these will build properly, up to and including the newly-wrapped
  searchbar sources.