disabled un-implemented items in main window
allow setting of per-pad properties

using the 'pad::property' notation
use "audio.gain" rather than "GAIN"
use "again" rather than "gain" so we know which one...
allow setting of audio and video-delay

both are given in [ms]
GUI now says that video-delay is in [ms] as well
implement basic video-mute (aka blackout)

it assumes that we are using `videomixer`;
LATER check whether this will work on the target machine

https://github.com/iem-projects/striem/issues/2


btw, it also mutes the audio
settable video delay
video delay (using frei0r)

i seem to remember that there was an easy way to accomplish a
videodelay with ordinary gst elements.

however `queue min-threshold-buffers=1` just freezes the pipeline...
adding audio parts of the delay-line
use videomixer for blackout

and a dummy videotestsrc as the blackout image
added delay & mute controls