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.
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.
Instead of relying on forward declared c++ classes from
gui in QgsLayoutItemRegistry, instead create a
QgsLayoutItemGuiRegistry which handles registration
of all the GUI specific behavior relating to layout items.
Remove all GUI related code from QgsLayoutItemRegistry.
This creates a cleaner split between core/gui code, and
given that there'll be a lot of gui specific behavior
which needs to be handled by a registry it makes sense
to keep this isolated in gui.
It also plays nicer with the sip bindings, which can't
handle forward declared gui classes in core.
c++ QgsLayoutItem metadata classes can directly register
a function which creates a QgsLayoutViewRubberBand for the item
subclass.
Python code cannot utilise this shortcut (due to inaccessibility
of forward declared gui classes from core Python classes), so
there's a separate gui registry utility class added for registering
prototypes for rubber bands for already registered item types.
Copy the same model as QgsMapCanvas uses, with separate
classes for individual interaction tools instead of keeping
all the logic in the QGraphicsView subclass (as is done
with composer)
