Allow mixed directions for slur/tie segments

[XiaoMigros]

Slurs and ties can now have individual direction positions for each segment. Changing it is currently mapped to 'flip horizontal' (Alt+X). The default behaviour has been left unchanged, for obvious reasons.

The properties panel shows indeterminate options when mixed directions are present:

It is intentionally not possible to modify individual segment directions from the properties panel.
Resetting the position option resets all the segments as well as the main spanner.

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