Bug 1075306 - Implement InputWindow (inherits from AppWindow)

- Instances of InputWindow replaces those iframes we werekeeping in InputFrameManager.
- InputAppTransitionManager has been eliminated, in favor of already-built AppTransitionController that works closely with AppWindow.
- Some IFM methods directly delegate to InputWindows.
- The coupling of KeyboardManager, InputFrameManager and InputWindows shall be resolved in a following bug where we'll implement InputWindowManager.
Couldn't we use this pattern?

if (config.isInputMethod) {
  // do 1.2.3.
} else {
  // do 4.5.6.
}