This setting allows per-map control of how close labels are permitted
to be placed to the map item's edges.
Sizes can be set using mm/inches/pixels/etc, and data defined
label margins are allowed.
Fixes#10314
Sometimes, calling some layout methods, results in sip being inable
to downcast the items to their correct type, resulting only
in a QgsLayoutItem object.
This works around the problem, albeit in an incredibly hacky way.
