Cleanup QgsProjectionSelectionWidget to use proper models to
drive the combo box. This removes a bunch of very fragile
logic regarding showing and hiding entries on demand, as it
allows us to move all the filtering logic to a single place
in a QSortFilterProxyModel subclass.
This has a few side benefits:
- The combos now dynamically respond to changes like recent
CRS being used in other places in QGIS
- The widget correctly respects horizontal/vertical crs filters
for all entries, including recent crs
