[Fronted] Significant refactoring of mixins for containers and buttons

WTD-839
Refactoring in advance of style mods to switcher when in frame;
Cleanup of markup to remove 'icon' class from invoke-menu elements;

platform/forms/res/templates/controls/menu-button.html

@@ -28,7 +28,7 @@
     <span class="title-label" ng-if="structure.text">
         {{structure.text}}
     </span>
-    <span class='ui-symbol icon invoke-menu'
+    <span class='ui-symbol invoke-menu'
           ng-if="!structure.text">
         v
     </span>