this commit adds 2 new features, a new event and a new property
The fallback property is usefull if you want to have a selection that always falls back
to some preconfigured things.
As a usecase, think about a list of languges where you can select the
languages you want to have. When everything is deselected, the list will
simply fallback to the preconfigured element (for example the language
The event is annoncing that there was a change to the selection. The
event is allowed to be defered, it will be emitted once for a range of
selection elements that happened during one loop iteration. This is
usefull if you are interested in a general selection where you want to
know that we changed from a specific set of selected element to another
set of elements, without monitoring every selectable in the widget.
Depends on D9661