When going from attribute mapping to model, it makes most sense
to go directly to the data tab, and when going from an action
name to the owner, we want to show the actions tab. Make it so.
Move away from cell editing, and use a popover instead. This makes
it easier to e.g. use a color chooser - there's just not enough room
in a cell for many things.
Much of this code is adapted from tests/prop-editor.c.
