Add ability to set boot layout.

src/style.css

@@ -545,6 +545,25 @@ button.playpen-header-button:hover,
   border-color: @pp_accent_strong;
 }
 
+/* -------------------------------------------------------------------------
+   The startup list
+
+   One card per tab the app opens itself with. The border is doing real work: a
+   row's parameter fields sit *under* its header line, and without something
+   drawing the boundary they read as belonging to the row below them instead. */
+
+.playpen-startup-row {
+  padding: 8px;
+  border: 1px solid @pp_border;
+  border-radius: 8px;
+}
+
+/* Indented under the header line, which is the other half of the same grouping:
+   these fields are a property of the layout picked above them. */
+.playpen-startup-params {
+  margin-left: 10px;
+}
+
 /* -------------------------------------------------------------------------
    The emoji picker