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.
fix #20947 : Glissando not angled between notes with the same pitch letter
try new design for voice selector
Merge pull request #362 from Soerboe/angled-gliss

fix #20947 : Glissando not angled between notes with the same pitch letter
resolve conflict for pull request #340
Merge branch 'trig-ger-midi_gui'
fix mac build
Merge pull request #339 from mgavioli/Fix_20703_keysig_changes_lack_naturals_ter

Fix #20703 - Key sig changes lack naturals (3rd solution)
Fix #21110 - Wavy glissando text position

- Text over a glissando wavy line collided with line.
- The wavy line occasionally collided with final note and was not always centred within the two notes.

Fixed in Glissando::draw() by:

- linking text vert. offset to line type
- always rounding down the number of wavy elements and positioning the line according to the number of actual elements
fix #21127, fix #21123, fix layout of square bracket
Fix #21121 - Pasting unstyled text causes crash

Fixed in ChordRest::drop() by checking for proper text style.
fix #21120: Increasing rest value causes corruption
Merge pull request #365 from mgavioli/Fix_21110_Wavy_glissando

Fix #21110 - Wavy glissando text position
Merge pull request #366 from mgavioli/Fix_21121_Pasting_unstyled_text_crashes

Fix #21121 - Pasting unstyled text causes crash
fix #20964: 7 and 9 tuplet groups result in a corrupted score when reopened
fix #20964: take 2