Split gstreamer plugin into smaller plugins providing fewer services

The gstreamer blob has been split into four plugins: audiodecoder,
camerabin, mediacapture, and mediaplayer.
Note: camerabin is still disabled because it is untested camerabin2
implementation.
A new qmake configuration use_gstreamer_camera has been introduced
and is needed for the mediacapture plugin to expose the camera
service. This configuration has been disabled by default.
Shared functionality has been moved to the internal gsttools library.

Change-Id: Ifb2604f440cfa97513d39f5d7978766c88eaec45
Reviewed-by: Michael Goddard <michael.goddard@nokia.com>