qt: log application messages from console.log and qDebug as VLC logs

This allows to get application message written through console.xxx in qml or qDebug logged as vlc messages. Qt internal messages are still sent through the default Qt log handler

this partially re-introduce !848 (closed), difference being:

• qt internal messages (now that we have --qt-verbose) are not sent to VLC, only application message are dispatched
• formatting and verbosity of application message is the responsibility of vlc logs
• 2 years after, having custom logger for qml never happened, I no longer expect this

verbosity rules have been moved to the new file mostly to reduce the quantity of inline code in qt.cpp

@fuzun