fix #253926: Key signature not displaying correctly on clef change at the start of a score

[mattmcclinch]

See Key signature not displaying correctly on clef change at the start of a score.

This is currently not an issue in the master branch, because the master branch does not currently allow "mid-measure" clef changes on the first tick of the measure. If this was an unintended consequence of the restructuring of code involving clef changes, I can submit a patch to bring back this ability.

[anatoly-os]

I like the code, but don't fully understand it. Could you please describe the logic of the for cycle?
On whether it was an intention or not completed refactoring in master, let's ask @wschweer.

[mattmcclinch]

This is the logic of the for loop:

Starting with the segment before the key signature, and working backwards, check the segmentType() of each segment. If it is a Clef segment, get the clef for the track that the key signature is on. Stop when one of the following occurs.

• A clef is found
• There are no more segments to examine
• The segment being examined is not on the same tick as the key signature

[wschweer]

Its a bug if "mid-measure" clef changes do not work in 3.0. In 3.0 the position of a segment must be determinable by type and tick as Undo can not use the segment prev/next pointers to insert the segment at the right position anymore. Fo this clefs use two different segment types: one for the system clef and another for "mid-measure" clefs to allow both at the same tick position.