Nested Node Groups backwards navigation keybinding

[j-jasz]

When inspecting nested nodes it could be nice to have a dedicated shortcut for moving up the tree.
Blender does that with Tab but it only works 1 level back and forth like a toggle. I think this design could be improved to move throughout the entire tree.
I'm not sure about using Tab for this thou, so maybe a Backspace would be good choice for this?

[Keavon]

#3283 and this are duplicates (thanks for pointing it out @j-jasz) but that has more progress so we'll keep that one.