Allow changing of selected entities in Events::SELECTED

Problem/Motivation

For some reason, the $entities argument in WidgetBase::selectEntities() is empty when using a view widget. Thus, the entities in the event object dispatched in selectEntities() is kind of useless, as one cannot access the selected entities.

Hence it's a bit confusing why the event is dispatched in the first place. Shouldn't it allow to do something with the selected entities? Even more useful would even be to allow altering the selected entities in an event subscriber. However in the case described above, neither is possible.

Proposed resolution

Depends on the intended purpose of the event dispatching in WidgetBase::selectEntities().