Blender Git Loki

Geometry Nodes: Node editor header interaction with active modifier

This commit makes the way the selector in the middle of the node editor
header interacts with the active modifier much more intuitive.

- With no active modifier, or when the active modifier is a non-node
modifier, the "new" operator adds a new nodes modifier.
- With an active node modifier, the drop-down affects the modifier's
selected node group.
- With no active object, there is no drop-down in the header.
- The node editor's node group always updates, even when there is no
active modifier.
- Pinning disables the drop-down and keeps the same node group
selected, regardless of the context.

This is designed to work similarly to materials and material slots,
which should be familiar to any Blender user. One workflow not allowed
by this design is editing any geometry node group, regardless of the
context, but if that turns out to be an issue it should be solved in a
similar way for all node editors.