Add support for parentheses before/after chords.

This allows a chord or sequence of chords to be indicated as optional
or alternative, and avoids the need to add the parentheses as text
or extra chords.
Remove obsolete assignment to _userName
fix #21249
fix tick of grace note after note
fix #6535 : Impossible to tie to a grace note
fix #20785: No tabs displayed when importing a GP file
Merge pull request #375 from Soerboe/tie-grace

fix #6535 : Impossible to tie to a grace note
Merge pull request #374 from Soerboe/grace-after

fix tick of grace note after note
Merge pull request #372 from softins/optional-chord-indication

Add support for parentheses before/after chords.
fix #21221