Add the 'initial-identity' property, which inserts identity for
at startup for event passing, and replaces it with a new child
when the first buffer (and caps) actually arrives.
https://bugzilla.gnome.org/show_bug.cgi?id=599469
gst_pad_set_caps on the internal source pad always succeeds, because
caps propagate to the peer with buffers, not immediately. Using
gst_pad_peer_accept_caps properly checks whether the actual
sub-element can accept caps when they change.
https://bugzilla.gnome.org/show_bug.cgi?id=575568
fsselector is too close to selector, so import is as autoconvert. Not importing
the history because the directory is also called gst/selector and it fails.