Fix alignment of unstyled text.

Separate out the offset from the text alignment, and use the
text alignment to set the bounding box.
Small fixes
GUI panel for MIDI import
Automatically show MIDI panel on drop midi file
Fix tests build error
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.
Merge pull request #325 from trig-ger/midi_gui

MIDI Import Panel
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
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