Skeleton of geometry panel

[sfmig]

This PR follows the materials panel approach to create the geometry panel.

Main features

• It adds a subpanel for each geometry node group, and populates each of them with the nodes' sockets that are candidates for randomisation.
• The header of each subpanel is a button that shows the graph for that node group in the Geometry Node Editor.
• The panel updates for node groups of infinite depth (i.e., node groups that are various level deep inside node groups are also considered).
• Only the subpanels of node groups relevant to the currently active object are shown as enabled.
• If a geometry node group is not inside another node group, and is not linked to a modifier of the currently active object, a new modifier is created and the geometry node group is linked to it

Closes #15

[ruaridhg]

Looks much cleaner now and therefore easier to read!

[ruaridhg]

It's a bit unclear where this is used?

[sfmig]

removed! I like to add the notebooks to have them in the history but sometimes I forget to remove them

[ruaridhg]

LGTM