Convert TermControl to .xaml (#4729)

This commit removes all of the custom UI initialization code in
TermControl and replaces it with a xaml file. Some dead or reundant code
was removed as part of this refactoring.

It also fixes two (quasi-related) issues:

* The search box, on first launch, was offset by the scrollbar even if
  the scrollbar was invisible.
* The scrollbar state wasn't hot-reloadable.