data/meld.css: Reinstate some compatibility CSS for pre-3.20 GTK+

Author: kaiw

data/meld.css

@@ -17,6 +17,11 @@ link-map {
     background-color: @theme_bg_color;
 }
 
+/* Only for backwards-compatibility with pre-3.20 GTK+ */
+LinkMap {
+    background-color: @theme_bg_color;
+}
+
 .meld-vc-console-pane {
     border-width: 1px 0 0 0;
     border-style: solid;