Don't set any default values for source element GstPhotography interface
settings, source elements should have sane defaults themselves.
Also, setting scene-mode is tricky as it is a superset of other GstPhotography
settings. This might cause problem with defaults e.g. setting scene-mode to
'night' may configure flash-mode as 'on' by definition, and after that
we don't want to override this flash-mode setting with gst-camerabin-test default
value. Moreover, user needs have an option to set scene mode first and then force
some individual setting to a different value from the scene-mode definition.
https://bugzilla.gnome.org/show_bug.cgi?id=639841
Splits the bus handler into sync and async, keeping the
prepare-xwindow handling at the sync for faster handling and
moving the others to the async handler
