DevTools: Add support for adding folders to workspace with drag and drop (renderer part)

Source/devtools/front_end/sourcesView.css

Index: Source/devtools/front_end/sourcesView.css
diff --git a/Source/devtools/front_end/sourcesView.css b/Source/devtools/front_end/sourcesView.css
index cc0994af61fa40d5a14f950d735f22df168bf036..1a5875d9a337473bd7e07b6c9b4eb3b4ed35d33d 100644
--- a/Source/devtools/front_end/sourcesView.css
+++ b/Source/devtools/front_end/sourcesView.css
@@ -30,6 +30,7 @@
 
 #sources-panel-sources-view {
     flex: auto;
+    position: relative;
 }
 
 #sources-editor-container-tabbed-pane {
@@ -62,3 +63,8 @@
 #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;
+}