QgsCoordinateReferenceSystemRegistry
This was the last bit of untested, direct sql database access relating
to custom projections. Now everything is nice and central and protected
by unit tests.
to QgsApplication
In the long term all methods for retrieving available CRS details
should be moved here (instead of being scattered all over the place,
as they are now). But for now the logic for saving and updating
user CRS definitions has been moved here only.
The initial motivation is to create a central place where objects
can connect to in order to listen for when a user makes changes
to their custom projections.
Refs #40704
