remove .gif and .xpm image formats

at least .gif doesn't work anyway.
fix #21932
use VoiceList typedef throughout code
add testfile for #21932
Fix string cursor in TAB note entry.

Recent commits broke the 'string cursor' update for 'string-above' and 'string-below' actions in TAB note entry,

Fixed.
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.
TAB: letter shortcut for "fret-8" action changed from 'I' to 'J'

The letter shortcut for the "fret-8" action was 'I' and this created a conflict with the "instruments" action.

It has been changed to 'J' as:

*) J is not used by TAB and is them free
*) there is no conflict with "enharmonic-up", as enharmony makes no sense in TAB's and its actions are disabled in TAB note entry.

(Background: TAB note entry has double shortcuts for fret marks: a letter series and a digit series. TAB's can use both letters and numbers and having both shortcuts makes user life MUCH simpler)
fix mtest
Merge pull request #515 from mgavioli/fix_string_cursor_in_tab_note_entry

Fix string cursor in TAB note entry.
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.
Merge pull request #517 from mgavioli/TAB_shortcut_change_for_fret-8_action

TAB: letter shortcut for "fret-8" action changed from 'I' to 'J'
Merge pull request #514 from lvinken/21932-strings-as-voice-names

21932 strings as voice names
Merge pull request #513 from Jojo-Schmitz/jpeg

remove .gif and .xpm image formats