Fix custom key signatures
Looking at the code I see that for all custom key signatures we set a C key. which means that playback will not work properly, and also updating the accidentals, moving the keys up and down or transposing them would not work correctly.
I'm making this post to gather ideas about how these could be fixed.
If we were to fix key signatures only for the symbols that have TPC values (flat, double flat, sharp, double sharp), we could start storing the list of notes for each key instead of the key name itself (just how we store them for the standard keys in tpcByStepAndKey in pitchspelling.cpp). Then all operations would have to use the list of notes instead of the key itself. We could have KeySig support both versions so that we don't need to do all changes at once.
I don't know what a good solution would be if we would want to support all accidentals in the master palette list. We might have to add them to the TPC list.