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.
I added this extra guard as the member accesses below this were giving me Gij errors [*] that I couldn't reproduce locally despite various efforts.

[*] https://treeherder.mozilla.org/ui/#/jobs?repo=gaia-try&revision=69c90449a1c1 Gij-3