[MSBuild] Add version defines for builders
[MSBuild] Set DefaultToolsVersion on build engines
[Ide] Show real GTK# about info on Windows

And nothing elsewhere. Assembly version is useless and confusing.
[Ide] Implement Tools->Edit file for sln files
[CBinding] Fix NRE with single-file projects
[IDE] Streamline Tools->Edit for proj and sln.
[Ide] Bug 17135 - mdtool crashes with NullReferenceException

Gtk.Settings.Default can return null if there is no default screen, e.g.
running from a remote terminal. In this case, don't include the Gtk theme
in the version description.