Added addition types to the user plug creation menu.

These now match the types available in the CompoundDataPlug UI.
Made SceneReadWriteTest refresh count management reusable.

So we can use it to avoid the same sorts of clashes in other test cases.
Added tag filtering to SceneReader.

Fixes #720.
Merge pull request #721 from johnhaddon/moreUserPlugTypes

Added additional types to the user plug creation menu.
Merge pull request #723 from johnhaddon/sceneReaderTagFiltering

Scene reader tag filtering