Add video-source-filter property that can be used to inject application
specific gstreamer element to camerabin pipeline. The video-source-filter
element will process all frames coming from video source.
One could add image analyzers to collect information about the stream,
or add image enhancers to improve capture quality, for example.
* decouple image capturing from image post-processing and encoding
* post image-captured message after image is captured
* post preview-image message with snapshot of captured image
Don't allow setting filename via img-done signal parameter but force app
use filename property. Don't stop capture when setting filename property.
Update check unit test based on the change.
