If another staff is linked to a TAB staff, entering the 2nd digit of a 2-digit fret number has no effect in the linked staff.

Fixed by iterating the fret no. change on all the notes linked to the note being edited.

Note: is it reliable to assume that elements linked to a Note are also Note's?
Merge pull request #474 from mgavioli/obey_multidigit_fret_nos_in_staff_linked_to_tab

Obey multi-digits fret nos. in staves linked to a TAB staff