I am going to misuse this bug a bit.  I've been working on implementing show-expanders and level-indentation properties, and figured that those are basically enough to get grouping working.  Showing groups would work like:
  * set show-expanders to FALSE
  * set level-indentation to something nice, say 20
  * make sure the group headers get a different cell background.

What's still needed is a way for a cell to span multiple columns in specific rows,  I am not really sure yet how I am going to implement that.

Created attachment 60719 [details] [review]
first take at a patch

First try at a patch.  The allowed range of the level-indentation property needs some thought (do we want to accept a negative indentation here for people who complain that the default indentation (which equals expander-size) is too large?).

There's one small commented out piece which needs research, will get to that later on.

Committed.  The "column spanning cells" chunk will be addressed later on (there's a separate bug open on that).