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.
Actually nah ;) "one" begins with the consonant sound /w/ (a semivowel, more precisely) so we don't use "an" for it. It's like we should say "an hour" instead of "a hour" since "h" is silent there.

Feel free to take a look at the wiki page for some special usage if you're interested: http://en.wikipedia.org/wiki/English_articles#Distinction_between_a_and_an