Render rest in a <tabGrp> (tablature)

[reinierdevalk]

Is your feature request related to a problem? Please describe.
Currently, in tablature rests can only be encoded by means of a rhythm flag with no note -- i.e., by means of a <tabGrp> containing ONLY a <tabDurSym>. However, in some cases, actual rest symbols on the staff are used (as in a mensural notation); this can be with and without rhythm flag. See the facsimile example attached for an example of a rest without a rhythm flag; the MEI example is also attached.

Describe the solution you'd like
It should be able to encode and render these on-staff rests. This can be done simply using <rest> with the @altsym attribute, where the @altsym points to a user-defined symbol.

Additional context
See facsimile and MEI example:
verovio-issue_example-rest.zip