Bug 1042563 - [Homescreen] Implement performance startup events

https://bugzilla.mozilla.org/show_bug.cgi?id=1042563
@KevinGrandon, this should be the first statement in the `App` constructor (line 10) since it's the earliest moment we can dispatch `moz-chrome-dom-loaded` and no new app chrome elements are being added to the DOM after that. Statements after line 10 support event binding which is tracked by the `moz-chrome-interactive` event emission on line 34.