Fix #20096: Chord name navigation

Provides the following navigation in edit mode for Chord Names:
Tab/Shift-Tab - start of next/previous measure.
Space/Shift-Space - next/previous note, rest or chordname.
;/: - next/previous beat, according to time signature.
Ctrl-1..Ctrl-9 - move forward by specified duration (like f.b.).

There are still issues of layout and display to address, but they
will be approached separately.