For now it's only for WMS but you get the idea.
There is a new abstract base class for the source select
dialogs, that will grow with common behavior for all
the select dialogs.
Signals are forwarded from the (root) data items to the
app and then delivered to the various browser instances
and to the unified layer dialog.
A change in one of the browser items should trigger a
refresh in all the other browsers and dialogs.
