DevTools: [SSP] Implement adding new rule in user stylesheet

Source/devtools/front_end/sidebarPane.css

 /*
  * Copyright (C) 2006, 2007, 2008 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Anthony Ricaud <rik@webkit.org>
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
@@ skipping 176 matching lines @@
     vertical-align: bottom;
 }
 
 .hidden-placards-message {
     text-align: center;
     font-style: italic;
     padding: 4px;
     color: #888;
     background-color: #FFFFC2;
 }
+
+.sidebar-pane-button-new-rule {
+    background-image: url(Images/paneAddButtons.png);
+    background-position: 0 0;
+    height: 16px;
+    width: 22px;
+    float: right;
+    visibility: hidden;
+}
+
+.styles-section.matched-styles:not(.read-only):hover .sidebar-pane-button-new-rule {
+    visibility: visible;
+}
+
+.sidebar-pane-button-new-rule:hover {
+    background-position: -23px 0;
+}