The previous commit fixed that the dock widget wasn't
getting focused on tab change, but didn't fix the signal.
When changing tabs, FocusScope::setFocused() would bailout
early because it was already focused, so never emitted
the signal for the new focused dock widget.
Refactored the code a bit and it's more robust now.
Setting the current focused widget is now centralized in
DockRegistry. Before it was split in two different places.
Fixes issue #188
693fb84a3e