Fix exception when clicking Manage button.

Ignore non-MSBuild based projects when the Manage button is
clicked in the Manage Packages dialog.
Merge branch '4.x'

Conflicts:
	src/AddIns/Misc/PackageManagement/Project/Src/PackageManagementSolution.cs
	src/AddIns/Misc/PackageManagement/Test/Src/PackageManagementSolutionTests.cs