aboutdialog: Set a style class

Add the .aboutdialog style class to toplevel
widget of about dialogs, similar to what we
do for e.g. GtkAssistant.

gtk/gtkaboutdialog.c

@@ -101,6 +101,12 @@
  *                        "title", _("About ExampleCode"),
  *                        NULL);
  * ]|
+ *
+ * # CSS nodes
+ *
+ * GtkAboutDialog has a single CSS node with the name window and style
+ * class .aboutdialog.
+
  */
 
 typedef struct

gtk/ui/gtkaboutdialog.ui

@@ -2,6 +2,9 @@
 <interface domain="gtk40">
   <template class="GtkAboutDialog" parent="GtkWindow">
     <property name="resizable">0</property>
+    <style>
+      <class name="aboutdialog"/>
+    </style>
     <child type="titlebar">
       <object class="GtkHeaderBar" id="headerbar1">
         <child type="title">