fix #290356: Allow copy-pasting of the LetRing, PalmMute and Vibrato elements.

[dnovikov]

Resolves: https://musescore.org/en/node/290356

This code converts HairPin, LetRing, PalmMute and Vibrato to spanners, to utilize setTick() and setTrack2() methods. This prepares the ground to be used for any line element.

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[MarcSabatella]

That sounds like an ambitious goal, but surely there is a much simpler and a safer / more backwards-compatible fix for the short term? Shouldn't it just be to add the appropriate handler to a switch statement?

[dnovikov]

@MarcSabatella thank you for pointing out the right direction. I compiled the code with the switch statement only, and it worked fine.

[MarcSabatella]

Nice! I actually had no idea where the switch was :-), but I figured it had be something like this.