+ indicator centered in the dock when empty

[DivadNojnarg]

Use case: BristolMyersSquibb/blockr.dock#20.

For an interface that utilises the + right header button, it makes sense to ensure that the dock maintains at least a single empty group. This could be activated via an option in dock_view like allowEmpty = FALSE

[DivadNojnarg]

That said, we can also create a new group with add_panel on an empty dock :)