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
Set the tightbbox for unparsed chords.

... to ensure that they get spaced correctly.
fixing 2 mistakes in instruments.xml

and instr_language.xml
fix #20970 : Glissando collides with elements before note
Merge pull request #357 from Soerboe/fix20970_glissando-acciendental

fix #20970 : Glissando collides with elements before note
Merge pull request #356 from Jojo-Schmitz/instruments.xml

fixing 2 mistakes in instruments.xml
Merge pull request #349 from softins/layout-unparsed-chords

Set the tightbbox for unparsed chords.
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.
gray out inactive icons
colorize toggled icons
try something new for breaks design
fix crash in text indent more, ordered list
add simple system bracket
missing icon in Style->Text
tune icon color
make breaks and spacer bigger
fix overlap of lines in break symbols