Fix #20703 - Key sig changes lack naturals (3rd solution)

When a key sig changes, no natural is ever displayed, regardless of the "Hide naturals" property of the key sig.

Corrected by:
- adding methods to KeySig to locate the prev and next key sigs and update naturals accordingly when a key sig is added or removed
- in Score::addElement(), Score::removeElement() and undo's ChangeElement::flip() by calling the relevant KeySig method

Notes:
1) Covers updating naturals in following key sigs, if a previous key changes.
2) Fully integrated in undo
3) The fix is already predisposed to cope with a score style setting for natural display style: none, before, after accidentals. Until this style is implemented, the relevant code is commented out.
Updated MIDI import panel
Fix MIDI instrument names
MIDI import operation: dots instead of ties
Fix a bug in Measure::createEndBarLines().

A typo in function Measure::createEndBarLines() was creating a problem (additional bar lines not created when a mensurstrich span style was reduced at a normal span style).
At the same time, the typo was hiding another, perhaps more serious, bug: in some cases, extra bar lines were created for staves spanned by multi-staff bar lines.

Corrected.
pressed buttons have white icons now
update note duration icons
add minNoteDistance between note and accidental of next note
add portability hack for tweaked notes
Change Aeolus default to Equal Temperament

The default Aeolus temperament was Mean Tone which sounds out of tune to
our modern ears. I have changed the default to 5 (Equal Temperament) and
added information on how to change this at the head of the file.
On a new installation, "Preferences" crashes

On a new (Linux?) installation, if no sound output is enabled, opening "Preferences" crashes.

In mscore/preferences.cpp, mscore/musescore.cpp, mscore/scoreview.cpp seq is accessed but set to 0.

Added some safeguard checks.

(Same for master as another recent patch for qt5)
fix #17451: Jazz Big Band and Jazz Combo Templates crash MuseScore
transpose & render unrecognized chords
token-based rendering & other improvements
simplify and improve parsing and rendering of parens and commas
add hidden style option to suppress loading of chords.xml
add facility to define rendering by token in chord description file
add class attribute handling to token declarations
save token definitions as part of chord list
revert Jazz Lead Sheet to go back cchords_muse for now