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)
Update version number for all stock plugins

Version number of stock plugins should match the editor version since
they're maintained as a unit.  Also enlarged editor prefs dialog as the
plugin list was rather cramped...
Convert image editor to plugin

Additionally moved history manager into Sphere.Core.Editor and
HistoryPage subclasses into their respective plugins
New editor registration support

This allows all image editing (including tiles in the map editor, etc.)
to be handled in one place--the image plugin.  Also started overhauling
the plugin system.  Instead of passing the Host all over the place,
plugins can use methods in the PluginManager class.
Merge pull request #20 from fatcerberus/master

Submitting map plugin for further testing