Bug 552852 - [Categorization] ViewModelContext leaks in tab renderings

- ensure that the renderer used to render the contents of a tab is finalized so that it will hook its dispose listener to remove itself as a view model context user
- enhance existing unit tests with assertion of view model context user clean-up

Change-Id: I278ac8043a676a63d2218ccb098fff408f42efae
Signed-off-by: Christian W. Damus <give.a.damus@gmail.com>
3 files changed