Various changes

A couple of small things.
1) Removed some old unused properties in the Tag and TagGroup
viewmodels.
2) Changed the way adding/removing an item to/from a TagGroup works
3) Added a ViewModelLocator class that can find ViewModels by providing
the Model they are associated with.
4) Improved the selection of Tags/TagGroups. Selecting a TagGroup now
selects all Tags in that group, and selecting all Tags in a TagGroup
will mark that TagGroup as selected.