Doc: Copy snippets from the Qt Multimedia module

Extract a compilable subsection of...
- src/multimedia/doc/snippets/multimedia-snippets/camera.cpp
- src/multimedia/doc/snippets/multimedia-snippets/video.cpp

...into the snippet folder searched by:
- src/multimediawidgets/qcameraviewfinder.cpp
- src/multimediawidgets/qgraphicsvideoitem.cpp
- src/multimediawidgets/qvideowidget.cpp
- src/multimediawidgets/qvideowidgetcontrol.cpp

Change-Id: Ica3c21b65bc57d5686a594afca09e771cc376cc0
Reviewed-by: Jerome Pasion <jerome.pasion@digia.com>