parse-launch: Support linking all pads with new operator

parse-launch: Support linking all pads with new operator

Introduce a new operator ':' - e.g. element1 ':' element2

For example, 'uridecodebin : encodebin' -
if the encodebin has multiple profiles compatible with the
decodebin, multiple links will be created.

With '!' , after one delayed link is successfully done, the
pad-added callback is disconnected.

https://bugzilla.gnome.org/show_bug.cgi?id=751450