composition: Add entry to the the hashtable before connecting to pad-added

Avoiding races where the pad would be added right between the connection
and inserting to the hashtable
composition: Check if we have an entry before trying to get its probeid

The object can be removed at the time the pad is removed, avoid segfaulting
when that happens.
gnlcomposition: grammar (childs -> children)

https://bugzilla.gnome.org/show_bug.cgi?id=701647
gnloperation: check for request pads on the element class not the factory

When checking for request pads as fallback, just look at the
element's class, not the factory, since there might not be
a factory (in case of python elements) or the factory might
be the wrong one (in case of a GstBin sub-class) and doesn't
have complete information.

https://bugzilla.gnome.org/show_bug.cgi?id=582244
build: add subdir-objects to AM_INIT_AUTOMAKE

Fixes warnings with automake 1.14

https://bugzilla.gnome.org/show_bug.cgi?id=705350
gnlcomposition: Drop any buffer flowing from a source when updating pipeline

There is a race where we can have an EOS from the element which
srcpad is our ghost pad but other elements upstream are still outputing
data and we flush the element so its pads have no segment info
anymore, and we end up having warning about flow before segement event.

The solution here is that we do not let any data flowing when we are
updating the pipeline.
composition: WIP to have a separate probe to drop data.