build: robustify Qt plugin- and qml-path detection

For Qt5 plugin-path detection The qt_helper worked well only for
directory layouts as provided by Qt installers and failed for
layouts provided by distros (Debian/Ubuntu/Opensuse). This resulted
e.g. in undetected qml-box2d plugins provided by distro packages.

Now we try different locations to account for directory layouts of
above mentioned distros. Might need to be extended for other distros
...