menu: add title theme options (#2097)

Add theme options `menu.title.text.color` and `menu.title.text.justify`.

Add font place MenuHeader: `<font place="MenuHeader">`

Add `Oblique` font style

```
<theme>
  <font>
    <slant>Oblique</slant>
  </font>
</theme>
```

docs/labwc-config.5.scd

@@ -448,6 +448,7 @@ extending outward from the snapped edge.
 	- ActiveWindow - titlebar of active window
 	- InactiveWindow - titlebar of all windows that aren't focused by the
 	  cursor
+	- MenuHeader - menu title
 	- MenuItem - menu item (currently only root menu)
 	- OnScreenDisplay - items in the on screen display
 	If no place attribute is provided, the setting will be applied to all
@@ -460,7 +461,7 @@ extending outward from the snapped edge.
 	Font size in pixels. Default is 10.
 
 *<theme><font place=""><slant>*
-	Font slant (normal or italic). Default is normal.
+	Font slant (normal, oblique or italic). Default is normal.
 
 *<theme><font place=""><weight>*
 	Font weight (normal or bold). Default is normal.

docs/labwc-theme.5.scd

@@ -67,6 +67,10 @@ labwc-config(5).
 	Vertical padding of menu text entries in pixels.
 	Default is 4.
 
+*menu.title.text.justify*
+	Specifies how menu titles are aligned in the titlebar.
+	Type justification. Default Center.
+
 *menu.overlap.x*
 	Horizontal overlap in pixels between submenus and their parents. A
 	positive value move submenus over the top of their parents, whereas a
@@ -178,6 +182,9 @@ elements are not listed here, but are supported.
 	Menu title color. Default #589bda.
 	Note: A menu title is a separator with a label.
 
+*menu.title.text.color*
+	Text color of separator label.  Default #ffffff.
+
 *osd.bg.color*
 	Background color of on-screen-display. Inherits
 	*window.active.title.bg.color* if not set.

docs/menu.xml

@@ -61,6 +61,12 @@
 </menu>
 
 <menu id="some-custom-menu">
+  <!--
+    Creates menu title.
+    To create an empty header with no text,
+    set label=" ", not label=""
+  -->
+  <separator label="custom menu" />
   <item label="Reconfigure">
     <action name="Reconfigure" />
   </item>

docs/rc.xml.all

@@ -43,6 +43,12 @@
       <slant>normal</slant>
       <weight>normal</weight>
     </font>
+    <font place="MenuHeader">
+      <name>sans</name>
+      <size>10</size>
+      <slant>normal</slant>
+      <weight>normal</weight>
+    </font>
     <font place="MenuItem">
       <name>sans</name>
       <size>10</size>

docs/themerc

@@ -63,6 +63,8 @@ menu.separator.padding.width: 6
 menu.separator.padding.height: 3
 menu.separator.color: #888888
 menu.title.bg.color: #589bda
+menu.title.text.color: #ffffff
+menu.title.text.justify: Center
 
 # on screen display (window-cycle dialog)
 osd.bg.color: #e1dedb