Multi-widgets with no control can no longer be added

[Denis-de-Bernardy]

Ran into this while updating and testing the #9450 patch.

The search widget, when inserted into a sidebar, no longer sticks, from lack of a save button.

Either it should get a save button so as to make it stick in its sidebar, or the sidebar should be saved when a widget gets inserted regardless of whether it gets saved or not.

The second approach is probably preferable (along with a save widget event, in case the user likes the defaults). The current way to do things can easily mislead a user into thinking that his widget was added and configured upon adding it to the sidebar -- when neither happens.

[azaozz]

(In [11176]) Fix widget option saving on drop, fixes #9721