added a calling dialog. when someone gchats you that you aren't already talking to, the calling dialog comes up to ask if you want to switch over to having a conversation with that person. I had to make a CustomEventLabelButton because the way that regular ofxUILabelButtons propagate events up to the ofxUICanvas and then the ofxUICanvas propagates them back down to all the widgets meant that destroying an ofxUICanvas in a callback to its newGUIEvent would result in seg faults because after you callback the ofxUICanvas will still be trying to call event callbacks on its children