Enhanced functionality for Central Package Management (ie PackageVersion)

[tintoy]

Some enhancements to the work started in #84:

• Only offer PackageVersion element completion if the MSBuild project's ManagePackageVersionsCentrally property is set to true (or True).
• Consider also hiding Version attribute completion on PackageReference elements if ManagePackageVersionsCentrally is set (might want to make this configurable though, if we choose to do it at all).
• Rename HandlePackageReferenceElementCompletion and HandlePackageReferenceAttributeCompletion to HandleElementCompletion and HandleAttributeCompletion since the handler now deals with multiple item types.