Add layout dialog helper.

src/style.css

@@ -529,6 +529,35 @@ dnd.playpen-tab-drag {
   color: @pp_err;
 }
 
+/* A directory parameter's field, and the completions hanging under it. The list
+   is a hint rather than a menu — it never takes the focus — so it is styled
+   quietly: the surface it sits on, and rows that only stand out when they are
+   the one the arrow keys have landed on. */
+.playpen-path-list contents {
+  padding: 4px;
+  min-width: 240px;
+  background-color: @pp_surface_raised;
+}
+
+.playpen-path-list list {
+  background-color: transparent;
+}
+
+.playpen-path-list row {
+  border-radius: 6px;
+}
+
+.playpen-path-row {
+  padding: 3px 8px;
+  color: @pp_text;
+}
+
+.playpen-path-more {
+  padding: 3px 8px;
+  font-size: 0.85em;
+  color: @pp_text_faint;
+}
+
 /* Each captured pane in the save dialog, so the sections read apart. */
 .playpen-dialog-pane {
   padding: 8px;