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

Source/devtools/front_end/sourcesView.css

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 57 matching lines @@
 }
 
 #sources-panel-sources-view .sources-toggle-pretty-print-status-bar-item .glyph {
     -webkit-mask-position: -256px -24px;
 }
 
 #sources-panel-sources-view .sources-toggle-pretty-print-status-bar-item.toggled .glyph {
     background-color: rgb(66, 129, 235);
 }
 
-#sources-panel-sources-view .drag-mask {
-    background-color: transparent;
-    z-index: 1000;
-}
-
 .source-frame-debugger-script {
     background-color: rgba(255, 255, 194, 0.5);
 }
 
 .source-frame-unsaved-committed-changes {
     background-color: rgba(255, 225, 205, 0.40);
 }
 
 .editor-container-unsaved-committed-changes-icon {
     background-image: url(Images/statusbarButtonGlyphs.png);
     background-size: 320px 144px;
     background-position: -202px -107px;
 }
 
 button.status-bar-item.scripts-debugger-show-hide-button {
     display: none;
 }
 
 button.status-bar-item.scripts-navigator-show-hide-button {
     display: none;
 }