Clean up DynamicText horizontal alignment.

Until now, DynamicText alignment was a mess:
 - in define-grobs.scm, X-offset property was initialized to
   ly:self-alignment-interface::x-aligned-on-self, but
 - Dynamic_engraver called set_center_parent on every created
   DynamicText, so that half a NoteHead width was always added
   to its X-offset (very confusing for the user),
 - DynamicTexts living in a Dynamics context used a completely
   different offset callback, which aligned them on NoteColumns.

Since aligned_on_parent is now able to correctly align grobs
with PaperColumn parents (issue 3254), we can use it for all
DynamicTexts and have a single interface for the job.

Expected changes in output: DynamicTexts in Dynamics context
aligned to suspended noteheads may be placed up to 1/4 NoteHead
width further to the right.  This shouldn't be a problem.