For overlays that do not need to do special things when a
window is dismissed, provide a default cancel function.
This simplifies implementation of the information-only
dialogs, such as the 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.
