figure out value of ipc-message 'event' parameter

[mykmelez]

atom_renderer_ipc inserts a null value into the list of values it passes to ipcRenderer event listeners when it receives an ipc-message. The value represents an event parameter.

It's clearly wrong to set event to the null value, although it works for the single message that atom_renderer_ipc currently receives, since the handler for that message ignores the event parameter.

But we should figure out what it's actually supposed to be.