This allows registration of item creation actions which create
items with an existing item type, but custom creation functions.
E.g. allowing actions for creation of ellipse/triangles/etc
from a single layout item type.
This allows the designer dialog to group the corresponding item
actions together (i.e. grouping all basic shape creation actions
together), but without any hardcoded special handling so that
plugin based items can also be grouped.
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.
