[FEATURE] Allow custom projections to be defined using WKT strings

instead of just proj strings

WKT strings are lossless and allow definition of a much wider range
of projections than are possible using proj strings.

A new combo box in the Custom Projections dialog allows users to
choose whether the new projection is defined using a WKT or PROJ string.
If possible, the current projection definition is automatically converted
when the combo box is changed by the user.

Also fixes #25918 in the process