Phase out Host property of plugins

Plugins can now access the IDE globally through the PluginManager class.
There's only ever one IDE instance per session, so this avoids having to
pass the host reference around.
Script editor plugin support

This allows the map editor to use the active plugin for script editing,
reducing bloat. The ScriptEditShim in Sphere.Plugins will fall back to a
generic textbox if no script plugin is available.
Massive codebase cleanup

Removed most of the dead code left over by plugin conversion, added some
more XML documentation, moved map and image menu items into their
respective plugins, various other misc. cleanup
Remove unused map forms from Sphere Editor

Missed these in the last commit.
Update .NET Framework version in About box

About dialog still said we were using .NET 2.0, updated it to say 4.5
Windowstyle and spriteset plugins

This completes plugin conversion. Now to wait for the regressions to
start popping up... :o)
Rename project

Solution is now properly called Sphere Studio instead of Sphere Editor
Project organization overhaul

Normalized all plugin names, corrected folder structure to match
solution, etc.
Hotfix for embedded script/image editors

EditShim-based editors should now work properly.
Fix menu visibility for plugins

Map, spriteset etc. menus are now properly hidden when document is not
active
Add missing XML doc comments

No more "Missing XML doc" warnings, yay!
Internalize classes

A bunch of classes were public that didn't need to be; most of them were
able to be marked internal.
Merge pull request #21 from fatcerberus/master

Script editor support, plugin conversion