Use aligned-on-x-parent instead of other callbacks for some grobs

Use aligned-on-x-parent instead of other callbacks for some grobs

Now that we have an easy way of specifying different alignments for
grob and its parent (see 0c4f221e5d), we can use aligned-on-.-parent
in places that we previously used .-aligned-on-self.  With this change,
the users have more control over the placement of grobs.

Affected grobs:
- InstrumentSwitch
- Script
- SostenutoPedal
- SustainPedal
- UnaCordaPedal