Store contexts as todos, not tags, step 5: much porting
Summary:
- Finish porting context repository
- Port all usage of GenTag (except in akonadicachingstoragetest)
- Port context queries
- Port LiveQueryHelpers::fetchItemsForContext
Reviewers: ervin
Reviewed By: ervin
Differential Revision: https://phabricator.kde.org/D19141