Fix #32396 : Bugs in TAB note inputs.

Several inconsistencies accumulated in TAB note input during recent updates.

The main point is that currently the pitch, fret and string of a note -- if all are known -- must be changed in a single step; otherwise, the fretting algorithm intervening after each step notices they are inconsistent and refrets (potentially) the whole chord, which leads to loosing the fretting entered by the user.

A specific `undoChangeFretting()` function has been added, to be used with TAB's instead of `undoChangePitch()` when all the new fretting data (pitch, fret and string) are known in advance.

Also, the fretting algorithm has been changed to be more conservative, i.e. to keep the current note fretting (potentially entered by the user) as much as possible.
Merge pull request #1283 from mgavioli/Fix_32396_tab_note_input

Fix #32396 : Bugs in TAB note inputs.