[Fronted] Significant refactoring of btn classes

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;

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

@@ -25,7 +25,7 @@
         <a href=""
            ng-click="ngModel.cancel()"
            ng-if="ngModel.cancel"
-           class="btn normal outline ui-symbol close">
+           class="btn normal ui-symbol close">
             x
         </a>
         <div class="abs contents" ng-transclude>