Add a WIX condition to prevent the installer from running on Windows XP

The minimum supported Windows version is now 6.0 (Windows Vista)
Merge pull request #605 from mono/xp-unsupported

Add a WIX condition to prevent the installer from running on Windows XP
Bug 20649 - 'System.ArgumentOutOfRangeException' error displayed on adding multiple encodings.
Bug 20653 - Newly encoding added doesn't render immediately at File> Save as dialog.
[Ide] Show a message instead of throwing when selecting an invalid project
file."
Updated reference to md-addins
Merge pull request #606 from mono/bug20288

[Ide] Show a better message for non-project selected items.
Bump MD version
Updated reference to md-addins
Merge branch 'master' into preview-l

Conflicts:
	version-checks