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

Source/devtools/front_end/sourcesPanel.css

Index: Source/devtools/front_end/sourcesPanel.css
diff --git a/Source/devtools/front_end/sourcesPanel.css b/Source/devtools/front_end/sourcesPanel.css
index a04ddcdc4104cc7b5f4e3e6de59c358e82fe6048..b9978e045d72dc510925f97c12cda5c7e2704762 100644
--- a/Source/devtools/front_end/sourcesPanel.css
+++ b/Source/devtools/front_end/sourcesPanel.css
@@ -201,3 +201,19 @@ a.worker-item:hover {
     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;
+}