Allow multi-digit fret entry also in TAB's using letters.

With TAB's using letters instead of numbers, entry of high frets using multiple digits in sequence was disabled.

Enabled, as there have been requests for it.
Merge pull request #516 from mgavioli/TAB_multidigit_note_entry_also_for_letter_tabs

Allow multi-digit fret entry also in TAB's using letters.