[Frontend] Manual re-do of collapse/expand panes

open #90
Added code to MCTSplitter and MCTSplitPane to allow
toggling of CSS class 'resizing' on pane elements when
the user is actively using the splitter;
Added resize transition animation to split pane elements
when user not actively resizing;

platform/commonUI/general/res/sass/helpers/_splitter.scss

@@ -36,6 +36,9 @@
                 border-color: $colorGrippyInteriorHover !important;
             }
         }
+        &:active {
+            background-color: pullForward($b, 5%);
+        }
 	}
 	&.horizontal {
 		// Slides vertically up and down, splitting the element horizontally