Fix alignment of unstyled text.

Separate out the offset from the text alignment, and use the
text alignment to set the bounding box.
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.
Fix shape and initial position.
Merge pull request #312 from softins/unstyled-text

Fix alignment of unstyled text.
minor/typo fixes
update MusicXML website in about box
On a new (Linux?) installation, if no sound output is enabled, opening "Preferences" crashes.

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

Added some safeguard checks.
fix #20997 fix #19149
fix #15943: Tuplet creation from empty measures (whole rests)
change text in staff properties dialog
fix regression: position of instrument names
experiment new color and thickness for breaks and spacer to make them more discrete
rename icons according to FreeDesktop Icon Naming Specification
change icon set
add align-vertical-baseline icon, fix alignment icons in Style->Text
fix resource file for Mac OSX
replace more icons
replace more icons, take 2
fix #11684: replace 'Display' by 'View' in the main menu
Fix #21040: closing the Synth Control window with its own close button leaves the "View | Synth" menu item checked.

Fixed by connecting SynthControl::closeEvent() with the relevant action's setChecked()