[Frontend] More sanding on label flex-box conversion

open #431
Moved object-label classes into their own include;
Fixing object-label in edit Elements pool and
Inspector; mixin refactoring;

platform/commonUI/edit/res/templates/elements.html

@@ -24,7 +24,9 @@
     <ul class="tree">
         <li ng-repeat="containedObject in composition">
             <span class="tree-item">
-                <mct-representation key="'label'" mct-object="containedObject">
+                <mct-representation key="'label'"
+                                    mct-object="containedObject"
+                                    class="rep-object-label">
                 </mct-representation>
             </span>
         </li>