[dashboard] Add `panelClicked` event.
[dashboard] Add `panelClicked` event.

I need this for AGDQ2016. It fires any time a panel's iframe is clicked in. `e.target` will be the panel iframe that was clicked.

nodecg/nodecg,madmatt/nodecg,madmatt/nodecg,IgorCandido/igrcnddStreamNotifications,IgorCandido/igrcnddStreamNotifications,nodecg/nodecg,nodecg/nodecg