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: 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
Add translations to notenames plugin

moving it into a folder of it own for this.

Remove superfluos generation of .pro files during realease step, they
are generated in the lupdate step and only used there.

Add a single file containing all supported locales and simply the
build/gen-*-projectfile* scripts to use this single source
Note 1: there's a difference in gen-qt-procectfile* v.s the others in
additionaly having an *_en.ts file
Note 2: in the bat files a hardcoded "..\" is used to find the locale
file whereas in the Shell-scripts "`basename $0`" is used. Is there such a
thing for .bat files too?
Note 3: the .bat files have been tested, the shell-scripts have not been
tested, although they should work.
Esp. the nested "`cmd ... \`cmd ...\` ...`" needs testing, using
"$(cmd ... $(cmd ...) ...)" would be much nicer, but is not available in
plain Bourne Shell
Note 4: the mechnism for plugins searches strings in qml and js files.
QML allows to call js files from withing a qml file, not sure whether
MuseScore supports that too, needs some checking. QML also allows to call
qml files from within a qml file, but MuseScore's Plugin Manager then
lists that other qml file as a plugin too. It doesn't load it though.

Adjust share/locale/Readme.md to above changes