output-selector: Add pad-negotiation-mode property

output-selector: Add pad-negotiation-mode property

Adds getcaps/setcaps to output-selector and adds a property
to select which type of negotiation should be done.

The available modes are:
 * none:   no negotiation (current behavior), getcaps return ANY and
           setcaps aren't set on any of the peers
 * all:    use all pads (default), getcaps returns the intersection of
           peer pads and setcaps is set on all peers
 * active: getcaps and setcaps are proxied to the active pad

https://bugzilla.gnome.org/show_bug.cgi?id=638381