Initial version of a toolbar for the node editor, as an alternative method for adding nodes.

Every node class has its own panel, so they can be collapsed individually. Nodes can be added from the tool bar by dragging & dropping onto the main editor area.

Still needs a lot of UI polish and some additional features to make it more useful, e.g.:

• Color themes for node classes for easier recognition
• Individual node type icons
• Search box for node types by name or tooltip
• Extra panel for favorite/often used node types

Current patch version can be found here: http://www.pasteall.org/27135/diff