This function will find all of the actions on from_widget in various groups and add them to widget.
As this just copies the action groups over, note that it does not allow for muxing items within the same group.
You should specify a key for mux_key so that if the same mux key is seen again, the previous muxings will be removed.
| widget |
a Widget |
| from_widget |
A Widget containing the groups to copy |
| mux_key |
a unique key to represent the muxing |