[Common UI] Refactor dialogs

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.

platform/commonUI/dialog/res/templates/overlay.html

@@ -1,8 +1,8 @@
-<div class="abs overlay" ng-show="ngModel.visible">
+<div class="abs overlay">
     <div class="abs blocker"></div>
     <div class="abs holder">
         <a href=""
-           ng-click="state.cancel()"
+           ng-click="ngModel.cancel()"
            class="btn normal outline ui-symbol close">x</a>
         <div class="abs contents">
             <div class="abs top-bar">