Bug 1075306 - Implement InputWindow (inherits from AppWindow)

- Instances of InputWindow replaces those iframes we were keeping in InputFrameManager.
- InputAppTransitionManager has been eliminated, in favor of already-built AppTransitionController that works closely with AppWindow.
- The coupling of KeyboardManager, InputFrameManager and InputWindows shall be resolved in a following bug where we'll implement InputWindowManager.
- Flow of showing/hiding/reusing/switching input apps & layouts has been streamlined.
Uh, I know what happens. It's coming from https://github.com/mozilla-b2g/gaia/blob/master/apps/system/js/app_window.js#L738
Please fix it in your patch, thanks.