Issue 1029: \thumb should behave like other fingerings

Issue 1029: \thumb should behave like other fingerings

This changes \thumb from a script to an actual bona-fide fingering.
The size tweak is a bit ugly: it might make some sense to resize the
actual glyph to match fingerings, but maybe it should then also be
relocated from "scripts.thumb".

[dak, 2012-12-13 17:43:58]

Don't tamper with font-size: it messes up \grace

[Keith, 2012-12-16 04:27:52]

Should be good.  The /total/ height of the glyph matches that of 1-5, without
changing too much from the previous size, so the height is a good choice.

There is a remote chance that someone has been overriding the script-definition
of thumb, but a note in 'changes.itely' would be enough. "The thumb indication
(mostly for cello) is now a Fingering rather than a Script."

[Graham Percival, 2012-12-20 23:57:59]

LGTM