KeySig key exposure not working

Although KeySig has its key() and setKey(Key) methods declared as Q_INVOKABLE they don't work from within the plugin framework. Reason is they work with the Key enumeration, which isn't exposed.

My proposed fix is to:
1. Expose the Key enum the same way as done for #96631: Expose text styles to plugins (thus base this new PR onto that one)
2. Expose a Q_PROPERTY named 'key' from KeySig with correct read and write functions.
3. Remove Q_INVOKABLE from key() and setKey() as those are (from the plugin framework pov) replaced by the property from item 2