qtcamera with gstreamer backend, encoding setting doesn't work

run /usr/share/qt5/examples/multimediawidgets/camera/camera,
switch video capture mode.
use "File"->"Settings" to change container, audio codec or video codec.
recording.
the setting not been taken effect. it always recorded as ogg+theoraenc+vorbisenc.

The root cause is that gstreamer camerbin need change to NULL state to refresh its internal encode pipeline. but QCamera only set to Loaded state which is READY state for camerbin. so the "video-profile" set by QCamera doesn't take effect in camerbin.

The patch attached set camerbin to NULL for gstreamer backend. it works fine on my side.