This adds inital Grease Pencil support for node tools.
Node tools work in `Object Mode`, `Edit Mode`,`Sculpt Mode`,
and `Draw Mode`.
While Grease Pencil has many editing tools, including editing
multiple frames at the same time, for now, node tools only
allow editing the current frame.
Currently, the idea is that node tools can do arbitrary changes
to the drawings, but cannot do changes to the existing layer tree, e.g.
changing the order of layers, removing a layer or groups, etc.
All the node tool specific nodes like `Selection` and `Set Selection`
are adapted to work with Grease Pencil. In `Draw Mode`, we currently
interpret everything as selected.
The `Active Element` node has a `Layer` mode that provides the
index of the active layer (if there is one).
When `Auto-Key` is used, a new keyframe is created on the
current frame.
Locked/invisible layers cannot be edited with node tools.
Pull Request: https://projects.blender.org/blender/blender/pulls/136624
a7d2b7850e