[Frontend] Markup and CSS for loading tree items

#445
New wait-spinner constants;
Markup fixed in wait-node.html;
Styles for .loading when applied to
.tree-item.wait-node;
Changed from percent-of-parent-
width sizing of spinner to fixed size;

platform/commonUI/general/res/templates/tree/wait-node.html

@@ -19,9 +19,4 @@
  this source code distribution or the Licensing information page available
  at runtime from the About dialog for additional information.
 -->
-<li>
-    <span class="tree-item">
-        <span class="icon wait-spinner"></span>
-        <span class="title-label">Loading...</span>
-    </span>
-</li>
+<li class='tree-item t-wait-node loading'>Loading...</li>