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
This controls how close labels are permitted to the edges of the map
item. The labeling engine will then try other candidate positions
in order to avoid placing labels within this margin.
This allows the CRS for map items to differ from the canvas/project
CRS. It also allows different map items to have different CRS,
eg an overview map can be set to a different CRS to the main map.
An unfortunate side effect of this change and the ongoing work
to separate compositions from canvas is that datum transforms
are no longer supported in composer. This cannot be fixed until
the datum transform store is rewritten to not depend on canvas
(ie, it's also broken for upcoming multi-canvas work)
