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 selection detection of colornotes plugin
fix warning, unused variable
fix #32166
Merge pull request #1276 from heuchi/heuchi-fixColornotesPlugin

fix selection detection of colornotes plugin
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