menu: support titles

...defined by `<separator label="">`.

Also add the theme option `menu.title.bg.color: #589bda`

The following will be added in separate commits
- menu.title.bg.border.color: #7cb6ec
- menu.title.text.color: #ffffff
- menu.title.text.justify: center

docs/labwc-menu.5.scd

@@ -25,7 +25,7 @@ The menu file must be entirely enclosed within <openbox_menu> and
   <!-- A submenu defined elsewhere -->
   <menu id="" />
 
-  <!-- Horizontal line >
+  <!-- Horizontal line -->
   <separator />
 
   <!-- An inline submenu -->
@@ -33,6 +33,9 @@ The menu file must be entirely enclosed within <openbox_menu> and
     ...some content...
   </menu>
 
+  <!-- Title -->
+  <separator label=""/>
+
   <!-- Pipemenu -->
   <menu id="" label="" execute="COMMAND"/>
 
@@ -63,6 +66,11 @@ The menu file must be entirely enclosed within <openbox_menu> and
 *menu.separator*
 	Horizontal line.
 
+*menu.separator.label*
+	In a "separator" element, the label attribute transforms the separator
+	from a horizontal line to a menu title (heading) with the text specified
+	by label in it.
+
 *menu.execute*
 	Command to execute for pipe menu. See details below.