Added titles to Node Graph context menus.
Implemented connection hiding for the Node Graph.

Fixes #429.
Fixed const correctness of GraphGadget::getNodePosition()
Replaced Enabled/Disable node menu item with Enabled checkbox.

This brings it in line with the new connection menu items. Also moved the implementation into NodeGraph.py so it can be reused by the config files for other apps.
Merge pull request #457 from johnhaddon/hiddenConnections

Connection hiding in the Node Graph