WTD-839
Refactoring in advance of style mods to switcher when in frame;
Moved btn styles from _controls.scss to _buttons.scss;
Markup cleaned up with css classing simplified;
Refactor overlay templates to separate out the overlay
container from the inner dialog containing the form;
this permits the easy reuse of that overlay container
to supply a non-form-like About dialog, WTD-667.
Refactor bundle platform/commonUI/dialog such that
the concerns of showing dialogs (including showing
a single instance thereof) and managing changes to
the DOM are more cleanly separated. This simplifies
testing and satisfies code style guidelines for
this bundle, in preparation for its inclusion among
common user interface bundles to be transitioned
in WTD-574.
