when using gst-launch with child-proxy its needed to be able to set properties on objects that are going to be created

for a practical example: with the patch one can do set the xpos/ypos for videomixer-pads on the commandline like this:

gst-launch videomixer name=mix sink_0::xpos=100 sink_0::ypos=100 ! ffmpegcolorspace ! xvimagesink \
\
videotestsrc pattern=1 ! video/x-raw-yuv, framerate=\(fraction\)10/1, width=100, height=100 ! videobox border-alpha=0 alpha=0.5 top=-70 bottom=-70 right=-220 ! mix.sink_0 \
\
videotestsrc ! video/x-raw-yuv, framerate=\(fraction\)5/1, width=320, height=240 ! alpha alpha=0.7 ! mix.sink_1