Renamed gaffer:visibility attribute to scene:visible.
Improved gaffer:visibility attribute backwards compatibility.

- Raw pointer rather than smart pointer for plugSet(). We only use smart pointers where we need to manage ownership.
- Slightly more robust way of identifying the relevant plug.
- Removed signal handling after the correction has been applied.
- Removed unnecessary connection member variable.
- Added \todo discussing a better mechanism which relies on doing #746 first.
Merge branch 'davidsminor-sceneVisibleAttributeName'