Revert "Actually removed custom debug class"

This reverts commit b0d6b8e2f0012fdead804b2f41f0a6c413ba685f.
Revert "Removed debug class again in favor of QDebug"

This reverts commit 0854d89651359721554f77f221b650dca40f8230.
Use CMake config.h.in to disable Debug output permanently in Relase

The CMake Platformcheck mechanism is used to create a config.h that
disabled the creation of debug output permanently in the release-target,
slightly speeding up everything.
Use Qt5 for unittests only

Because we won't be using QtQuick as GUI (for security reasons), we
removed Qt as run time dependency and only use QTest for building
unittest executables.