Fixed Bug 6620 - Cannot properly remove item from current solution

Bug happened because after deletion file, the solution was not save.
Merge pull request #593 from vitorelli/master

Fixed Bug 6620 - Cannot properly remove item from current solution
[MSBuild] Add extension point for global properties

Also unifies setting core global properties.
[MSBuild] Remote builder support for arbitrary target/item evaluation

Not exposed yet, but used for reference evaluation.
[Core] Fix recursive includes for custom MSBuild frameworks
[Core] Filter overridden assemblies from chains of framework includes

And fix up refreshing/reloading stuff as appropriate.
[Ide] Fix crash when loading pads

When registering a new pad as a result of the installation of a new add-in
the corresponding DockItem was not created. This is now done.

Fixes bug 20600 - Installing an addin that creates a new pad throws a null
reference exception and crashes Xamarin Studio.
[Core] Properly load projects with ids without curly braces

Fixes bug 20618 - Adding F# project fails to put project identifier in curly
braces in solution file.
Merge branch 'master' into msbuild-import-conditions

Conflicts:
	main/src/core/MonoDevelop.Core/MonoDevelop.Core.csproj