This implements the status bar as footer region in the
spreadsheet editor.
It shows the total number of rows and columns as well
as how many rows are actually visible (based on the filter).
The implementation stores the stats in a runtime struct during
drawing and the status bar reads from that struct.
Ref T86142.
Differential Revision: https://developer.blender.org/D10693
This adds the initial boilerplate code that is required to introduce
the new spreadsheet editor. The editor is still hidden from the ui.
It can be made visible by undoing the change in `rna_screen.c`.
This patch does not contain any business logic for the spreadsheet editor.
Differential Revision: https://developer.blender.org/D10645
Ref T86279.
