Trill spanners too short

[Tuukka Verho]

Version 2.9.20 produces too short trill spanners (or at least they seem
too short to me). The "trill spanner" regression test illustrates the
problem well enough. In addition, in cases when the spanner is so short
that no trill line is shown, the "tr" symbol is still left-aligned,
instead of centered as it should (like with \trill).

However, even with these problems I see this as an improvement to the
previous behaviour -- the tendency to extend the trill line over the
next note was very annoying in earlier versions. Thanks to the
developers for fixing that!

Tuukka Verho