This is only used by virtual layers, but it
might be handy for others too and now I can
start to remove all specialized source
select add methods from the datas source
manager dialog.
This is only the first step: next is looping
through the source select provider registry
to get all needed GUI elements for the dialog.
All signals are now in the base class, even if only
a subset of available providers actually emits them.
This way we can handle all source select dialogs
the same way, regardless if they are vector, DB
or raster (or others).
This modification was necessary because the current implementation
of the source select dialogs within the unified add layer dialog
create the provider dialogs the first time and do not destroy
them, this means that the canvas extent and CRS can change from
a dialog invocation to the next and the extent and CRS need to
be updated at layer creation time.
