Issue 3254: align unassociated lyrics using NoteColumn extent.

Issue 3254: align unassociated lyrics using NoteColumn extent.

This makes unassociated lyrics behave consistently with
associated lyrics.  Until now, "standalone" lyrics
were left-aligned (more precisely: their X-offset was 0);
changing self-alignment-X didn't have any effect on them.
Now it's possible to specify their alignment, like with
associated lyrics.

Also, this changes how issue 104 was resolved (see 0b14e8b2e122d) -
alignment of syllables that are associated to a context without
noteheads is no longer aborted - and solves issue 247.

Later on, these changes should allow using aligned_on_parent for
other grobs (such as DynamicTexts).

Expected changes in output: all lyrics should be centered by default.