DevTools: rename SourceEditor to SourceView, make entire SourcesPanel a drop target.

Source/devtools/front_end/sourcesPanel.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 183 matching lines @@
 }
 
 .panel.sources .sidebar-pane-stack {
     overflow: auto;
 }
 
 .threads-toolbar {
     padding-left: 10px;
     margin-top: -1px;
 }
+
+.panel.sources .drag-mask {
+    background-color: rgba(255,255,255,0.8);
+    z-index: 1000;
+}
+
+.panel.sources .drag-mask-inner {
+    font-size: 30px;
+    color: #999;
+    display: flex;
+    justify-content: center;
+    align-items: center;
+    margin: 20px;
+    border: 4px dashed #ddd;
+    pointer-events: none;
+}