Importer: add optional URL, zoom level and file extension in YAML

If a stack in a project file now contains a 'url' field, it is used as
the stack's image base. If done so, the fields 'zoomlevels' and
'fileextension' need to be available as well. If no 'url' field is
present, the importer looks for the 'folder' setting. If there are then
still 'zoomlevels' and 'fileextension' fields around, those will be used
as well. The importer will retrieve this information only if one of them
is missing.
Importer: add support for overlays

In a project file, each stack can now have a set of overlays
attached. In terms of declaring a data source, overlays behave very
similar to stacks, but don't need zoom level information. One can
 define a default opacity if wanted (if not, it defaults to zero).
Also, the confirmation view now displays all the overlays for each
stack.