fix #30266: Some button labels are not translated

Fixed by replacing QDialogBottonBox by QPlatformTheme, which seems what
Qt 5.2 (?) and 5.3 (!) needs as the context, to load the right
translations for these button labels.

Also some more cosmetic fixes for the German translation
Fix #31176 - Appearing and disappearing courtesy clef

When courtesy clefs are turned off, some sequences of steps involving clef changes across system breaks result in spurious courtesy clefs being shown. Also, occasionally, the layout oscillates between showing / hiding courtesy clefs and/or fitting its measure in the same system or the next system. See http://musescore.org/en/node/31176 for details and examples.
expose annotations of Segment to plugins
fix selection detection of colornotes plugin
Merge pull request #1276 from heuchi/heuchi-fixColornotesPlugin

fix selection detection of colornotes plugin
Bring Plugin save and load options and dialogs into line with score
saving and loading.
Correct plugin name default
Fix: don't apply key detection for drum tracks
Merge pull request #1280 from trig-ger/midi_fixes

Fixes for MIDI import
Merge pull request #1272 from Jojo-Schmitz/translation

fix #30266: Some button labels are not translated
Bar lines: fixing custom type and generated management

There are some inconsistencies in the current management of bar line `_generated` flag and user-modified type:

- Bar lines created by the New Score Wizard are flagged as non-generated, as well as bar lines of measures manually **appended** by the user, while bar lines of measures **inserted** are flagged as generated.
- If a generated bar line is individually changed of typed, it remains flagged as generated, it is not saved and the change is lost upon saving and re-loading.
- The management of the internal flag `BarLine::_customSubtype` is not always consistent.
- The `Measure::_endBarLineGenerated` flag was not always restored properly by undo.

This PR introduces the following fixes:

- The `_generated` flag is consistently used for bar lines whose type can be reconstructed from the context and then do not need to be saved to the output file.
- Normal bar lines are **always** created as generated: initially created by the Wizard, manually appended or inserted.
- Bar lines with custom type (i.e. different from the type which can be expected according to the bar line context) are always flagged as non-generated, ensuring the custom type is written to the output file.
- The `Measure::_endBarLineGenerated` flag is stored by `ChangeEndBarLineType()` and restore upon undo.
- Some test reference scores, based on the inconsistent bar line `_generated` flag, have been uniformed.

Notes:

- Tests about measure (and then bar line) appending, inserting and relative undo's are already included in the `tst_parts` test suite.
- Some inconsistencies remain in the management of custom bar line span and of system-initial bar lines: for the sake of simplicity, they will be dealt with in separate PR's.
fix #32116: [MusicXML] new-page on first measure shouldn't create one more page
fix #32356: Crash when pasting 'all' to new file
fix #31971: Staff names of hidden instruments appear
Fix main layout memory leak
Fix score font memory leak
Fix TabScoreView memory leak
Fix M_new_divis memory leak
Fix QFontMetricsF memory leak
Fix Element memory leak