Blender Git Loki

Spreadsheet: breadcrumbs and node pinning

This introduces a context path to the spreadsheet editor, which contains
information about what data is shown in the spreadsheet. The context
path (breadcrumbs) can reference a specific node in a node group
hierarchy. During object evaluation, the geometry nodes modifier checks
what data is currently requested by visible spreadsheets and stores
the corresponding geometry sets separately for later access.

The context path can be updated by the user explicitely, by clicking
on the new icon in the header of nodes. Under some circumstances,
the context path is updated automatically based on Blender's context.

This patch also consolidates the "Node" and "Final" object evaluation
mode to just "Evaluated". Based on the current context path, either
the final geometry set of an object will be displayed, or the data at
a specific node.

The new preview icon in geometry nodes now behaves more like
a toggle. It can be clicked again to clear the context path in an
open spreadsheet editor.

Previously, only an object could be pinned in the spreadsheet editor.
Now it is possible to pin the entire context path. That allows two
different spreadsheets to display geometry data from two different
nodes.

The breadcrumbs in the spreadsheet header can be collapsed by
clicking on the arrow icons. It's not ideal but works well for now.
This might be changed again, if we get a data set region on the left.

Differential Revision: https://developer.blender.org/D10931