Use nsIEventListenerService to listen for all keypress events

[1ec5]

components/avim.js currently handles all window onload events by loading an overlay that in turn loads content/avim.js. But what if we centralize keyboard input handle into one object using nsIEventListenerService.addSystemEventListener()? We could then leave content/avim.js for UI management alone. AVIM would end up using less memory per window and, once Electrolysis is finalized (#25), less memory per tab. There will be an e10s shim like the one for addEventListener().

nsIEventListenerService is available starting in Gecko 1.9.2 (Firefox 3.6).