Blender Git Loki

Fix #35388, grouped nodes not editable in properties ui.

The issue here was that the "active" material node depends on the editor context. Previously (< 2.67) there was only 1 edited node group possible globally throughout Blender, so the active material in the context could be resolved more easily. The solution now involves the active_viewer_key variable (first introduced for compositor viewer nodes in r56271, naming is a bit awkward but hard to change in DNA). This key defines the "last modified" node tree to resolve ambiguity of active context items. For single editors the result is the same as in 2.66, if multiple editors are used with different node groups the last modified tree is used.