XMPPCapabilities featureElementsForXMPPCapabilities API

Typically all you want to do in xmppCapabilities:collectingMyCapabilities: is add feature elements to the query, so passing in the query directly into the delegate method isn't very clean. Moreover it doesn't deal with multiple modules supplying the same feature elements.

featureElementsForXMPPCapabilities: allows you to supply an array of feature elements and also discards duplicate features automatically.