jamulus --version should show Qt version from dynamic libraries

[softins]

When running jamulus --version, it displays the Qt version of the host on which it was compiled. It should display the Qt version of the dynamic Qt libraries being used on the running system, the same as About Qt... does.

The version displayed by jamulus --version is currently taken from QT_VERSION_STR, which was set at compile time.

Originally posted by @softins in #3228 (comment)