Add a new Gimp::tool_item_ui_list, which is a GimpTreeProxy over
Gimp::tool_item_list. This allows us to use either a hierarchical
or a flat tool list in the UI, by setting the "flat" property of
the new list.
Use Gimp::tool_item_ui_list in GimpToolPalette, so that the toolbox
layout is affected by this choice.
Add a "Use tool groups" toggle to the toolbox preferences, and bind
it to the "flat" property of Gimp::tool_item_ui_list.
(cherry picked from commit 3cda972100)
9362e89b50