playsink: Use signal handler IDs instead of disconnecting/blocking by function

This is cleaner and faster.
typefind: Add variant=itu to the h263 typefinder caps

https://bugzilla.gnome.org/show_bug.cgi?id=700770
decodebin: Connect and disconnect the have-type signal of typefind before starting/shutting down
decodebin: Use signal handler IDs instead of disconnecting by function

This is cleaner and faster.
Start on adding stream combiners to playbin.
Fix *-stream-combiner properties to be objects.
Use custom stream combiners when setting up groups.
Only set always-ok on pads that have that property.