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.
Removed unnecessary include from PathFilter.h.
Enabled input connections to PathFilter "paths" plug.

This allows it to be promoted to box level, be driven by expressions etc. It's feasible for a very nefarious user to drive it with an expression which varies based on the "scene:path" context entry, in which case descendant and ancestor queries would be inaccurate, but a user that nefarious deserves what they get.

Fixes #704.
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
Merge pull request #725 from johnhaddon/pathFilterPlugPromotion

Path filter plug promotion