menu: Add diagrams to CSS documentation

gtk/gtkcheckmenuitem.c

@@ -51,6 +51,12 @@
  *
  * # CSS nodes
  *
+ * |[<!-- language="plain" -->
+ * menuitem
+ * ├── ...
+ * ╰── check.right
+ * ]|
+ *
  * GtkCheckMenuItem has a main CSS node with name menuitem, and a subnode
  * with name check, which gets the .left or .right style class.
  */

gtk/gtkmenu.c

@@ -85,6 +85,13 @@
  *
  * # CSS nodes
  *
+ * |[<!-- language="plain" -->
+ * menu
+ * ├── arrow.top
+ * ├── ...
+ * ╰── arrow.bottom
+ * ]|
+ *
  * The main CSS node of GtkMenu has name menu, and there are two subnodes
  * with name arrow, for scrolling menu arrows. These subnodes get the
  * .top and .bottom style classes.

gtk/gtkmenuitem.c

@@ -90,6 +90,12 @@
  *
  * # CSS nodes
  *
+ * |[<!-- language="plain" -->
+ * menuitem
+ * ├── ...
+ * ╰── [arrow.right]
+ * ]|
+ *
  * GtkMenuItem has a single CSS node with name menuitem. If the menuitem
  * has a submenu, it gets another CSS node with name arrow, which has
  * the .left or .right style class.

gtk/gtkradiomenuitem.c

@@ -64,6 +64,12 @@
  *
  * # CSS nodes
  *
+ * |[<!-- language="plain" -->
+ * menuitem
+ * ├── ...
+ * ╰── radio
+ * ]|
+ *
  * GtkRadioMenuItem has a main CSS node with name menuitem, and a subnode
  * with name radio, which gets the .left or .right style class.
  */