API - TagQuery helper methods

[Hendy]

Add public helper methods GetTagGroups and GetTags (in group)

The BackOffice makes these type of queries to render the tags tree - these would also be useful in the public API.

[Hendy]

Update LookTags to handle a reserved wildcard tag "group:*"

Add static methods on TagQuery:

• string[] TagQuery.GetTagGroups(string searcherName)
• LookTag[] TagQuery.GetTagsInGroup(string searcherName, string tagGroup)