Blender Git Loki

Use toolbar tabs for node categories instead of just a long list of
panels.

Patch by @Severin (with minor modifications by me). As discussed in
{D535} the node editor does not have real modal operator tools like the
3D view or image editors for instance, so it makes sense to utilise it
this way. Tabs really help in this area due to the large amount of node
types and categories. Further tweaks could be made later if the need
arises.