fix #14764

convert qDebug();abort() to qFatal() and if(cond) abort() to
Q_ASSERT(!cond), esp. for the benefit of Windows, where it is impossible
to set a breakpoint on the abort()
Also modernizing mscoreMessageHandler() and making it more informative
by adding file, line and function to the output.
changing all assert() to Q_ASSERT()

except for thirdparty/portmidi/
Merge pull request #737 from Jojo-Schmitz/14764-qDebug+abort-to-qFatal

fix #14764