Package: Undeclare VS2010 support

Pending #1
Package: Fix VS2012 compatibility

Extension now works in stock VS2012 installs.

Thanks to @jaredpar for providing
reference assemblies that make this
possible without requiring multiple
SDKs to build.

Still has an assembly conflict between versions
10 and 11 of ComponentModelHost; this is
caused by Microsoft.VisualStudio.Shell.11.0,
which we need for EnvironmentColors.
This will probably be fixed by #1.

Microsoft.VisualStudio.Shell.11.0 also conflicts
with types used by ExternalSettingsManager
from Microsoft.VisualStudio.Settings v10;
solving this involves a global alias.
This is only used for designer themes.

Platform.WindowManagement (also only used
for designer themes) is now loaded dynamically.

We still have version conflicts in the designer proc.
This may be caused by missing binding redirects
for XDesProc.exe.  (which may not be easily fixable)
Package: Remove unused WPF editor DLLs
Notifications: Properly fix null ref

The DTE.MainWindow getter throws if it doesn't exist yet.
Notifications: Find actual VS installation path

Code stolen from SLaks\Root-VSIX
Delay initial settings creation until MainWindow

Fixes notification issues