Upgrade to .NET 4.5

4.5 is an in-place upgrade with several breaking bugfixes, which means
building against 4.0 could cause issues on machines with 4.5 installed
(e.g. Win8).  Better just to build against 4.5.
Convert map editor to a plugin

Could use some major cleanup (plugin is a bit bloated), but it works
without crashing the editor, at least. :o)