Name added to the History panel and Snapshots treeview (issue #3446)

+ gap added in the File Panel's fast export tab, filter tab and around
History panel.

rtdata/themes/RawTherapee-GTK3-20_.css

@@ -139,6 +139,7 @@ grid separator.vertical, box separator.vertical {
     padding: 4px;
 }
 
+
 /* Frames in the toolbox. Not MyExpander frames. */
 eventbox.frame {
     border-color: #565656;
@@ -796,3 +797,11 @@ paned.vertical > separator {
 #RightNotebook #ToolPanelNotebook stack {
     margin: 4px;
 }
+
+#RightNotebook #HistoryPanel {
+    padding: 4px;
+}
+
+#RightNotebook scrolledwindow {
+    padding: 4px;
+}