Implement partial spanners over repeats & jumps

[bkunda]

This feature allows partial ties, slurs, and lyrics extension lines to be applied across repeat & jump marks.

The user will need to use only the existing functionality for adding ties, slurs, and extension lines, and MSS will determine whether the conditions are such that either regular or partial variants of these objects should be applied. In the case of ties, where there is ambiguity, a contextual popup will allow the user to make a choice.

Full design spec has been sent to @miiizen.

There is a playback component to this project, which will be raised separately for @RomanPudashkin.