When adding an element to a bin we need to propagate the GstContext's
to/from the element.

This moves the GstContext list from GstBin to GstElement and adds
convenience functions to get the currently set list of GstContext's.

This does not deal with the collection of GstContext's propagated
using GST_CONTEXT_QUERY.  Element subclasses are advised to call
gst_element_set_context if they need to propagate GstContext's
received from the context query.

https://bugzilla.gnome.org/show_bug.cgi?id=705579