Truncate long dropdown menu items (e.g. tags in the sidebar)

ref #391

framework/core/less/lib/Dropdown.less

@@ -33,6 +33,8 @@
       border: 0;
       background: none;
       white-space: nowrap;
+      overflow: hidden;
+      text-overflow: ellipsis;
       .box-shadow(none);
       text-align: left;
       font-size: 13px;