Add push/pop pending to Dialog

* Works just like text input widget, shows pending texture on head and
  foot
* Had to make body in front of head and foot so shadow doesn't get
  obscured by the foot having a background instead of being transparent
* Also improve TextInputWidget's push/pop pending code to only make DOM
  changes when transitioning between pending = 0 and pending = 1

Change-Id: I5c45cc6395c8b0af1731aae49a7c8be3159148e6
Merge "Add push/pop pending to Dialog"