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)
