It's not yet possible to attach to an external app running a qml
debugging server, because the server is only started on startup if an
env variable is set. Changing this requires action from Brisbane, but
even the current solution works for C++ apps with QML in them.

Task-number: BAUHAUS-585
Reviewed-by: dt

The new QML Inspector depends on DebuggerPlugin. Also added a dropdown menu into
the debugger toolbar from which the user can select the used debugging
language, e.g. C++ or QML.

no longer uses LocalAppRunConfiguration. QmlInspector now uses
the qmlviewer started by QmlProjectManager instead of starting
its own, and clicking the Start Debug button within a QML
project will switch to QML Inspect mode and start the inspector.