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.
This allows gui widgets to utilise native platform interfaces.
Additionally attaching the instance to QgsGui allows QgsNative
to become a QObject in future if required and for use
of signals/slots in the native interfaces.
