decodebin2: Mark streams as complete on CAPS event but don't block

decodebin2: Mark streams as complete on CAPS event but don't block

This allows the following use-cases to expose the group and pads
before an ALLOCATION query comes through:
* Single stream use-cases
* Multi stream use-cases where all streams sent the CAPS event before
  the first ALLOCATION query

Some cases will still make the initial ALLOCATION query fail though,
which isn't optimal, but not fatal (it will recover when pads are
exposed, a RECONFIGURE event is sent upstream and elements can
re-send an ALLOCATION query which will reach downstream elements).

https://bugzilla.gnome.org/show_bug.cgi?id=680262