[REVIEW REQUEST] Add support for an emptyText property in ComboBox

[Knut Arne Vedaa]

Scott Palmer wrote:
> What about consistency with TableView?

TableView has:

ObjectProperty<Node> 	placeholder
This Node is shown to the user when the table has no content to show.

ListView doesn't, but I think it should. It means "no items", and both 
TableView and ListView has an "items" list, thus it is analogous.

ComboBox could then just utilize its ListView's placeholder, instead of 
introducing another "empty" concept.


Knut Arne Vedaa