DevTools: add emulation toggle button and splash screen.

Source/devtools/front_end/inspector.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 529 matching lines @@
 }
 
 button.status-bar-item.toggled-on .glyph:not(.shadow) {
     background-color: rgb(66, 129, 235) !important;
 }
 
 button.status-bar-item:hover .glyph {
     opacity: 1;
 }
 
+button.status-bar-item:active .glyph {
+    opacity: 0.8;
+}
+
 button.status-bar-item:disabled {
     background-position: 0 0 !important;
 }
 
 button.status-bar-item:disabled .glyph {
     opacity: 0.5 !important;
 }
 
 button.status-bar-item.extension {
     background-image: none;
@@ skipping 2332 matching lines @@
 }
 
 select.drop-down-menu {
     border: none;
     -webkit-appearance: none;
 }
 
 .viewport-control-gap-element {
     color: transparent;
 }