xhtml_style: set a bottom margin for dialog action

When the page height is larger than viewport, the scrollbar hides the
button on chrome.  Also does the "Loading" status message on chrome.
By using a margin, we make sure buttons are not behind scrollbar or
status message and are easier to reach.

product/ERP5/bootstrap/erp5_xhtml_style/SkinTemplateItem/portal_skins/erp5_xhtml_style/erp5.css.css

@@ -752,6 +752,10 @@ span.search button .image {
   background-image: url('images/exec16.png');
 }
 
+.bottom_actions {
+  margin-bottom: 1em;
+}
+
 .document .actions {
   position: relative;
   float: left;