Describe the actions of the fillet tools is a generic way, so that the
same general pattern can be used for other tools that modify shapes on
the BOARD.
Basically, an "ITEM_MODIFICATION_ROUTINE" is defined, which is
configured and called multiple times, calling back to the EDIT_TOOL when
it modifies or creates an item.
The motivation here is to make it easier to slot in new line-based
tools like chamfer, extend and so on without having to redo the
complicated item, selection and commit handling each time, and keep the
core "routines" simple and decoupled from the EDIT_TOOL's
internals.
This also resolves#15094 because the new commit handling does the right
thing when items were "conjured up" for the fillet (e.g. when a
rectangle is decomposed into lines).
Fixes: #15094
John Beard