Refactor title height to use 'titlebar.height' from themerc

docs/labwc-theme.5.scd

@@ -47,6 +47,10 @@ labwc-config(5).
 	Vertical padding size, used for spacing out elements in the window decorations.
 	Default is 3.
 
+*titlebar.height*
+	Window title bar height.
+	Default equals the vertical font extents of the title plus 2x padding.height.
+
 *menu.items.padding.x*
 	Horizontal padding of menu text entries in pixels.
 	Default is 7.
@@ -202,11 +206,6 @@ with the respective titlebar colors. For example: "close-active.png"
 
 The handle is the window edge decoration at the bottom of the window.
 
-# DERIVED DIMENSIONS
-
-The window title bar height is equal to the vertical font extents of the title.
-Padding will be added to this later.
-
 # SEE ALSO
 
 labwc(1), labwc-config(5), labwc-actions(5)

docs/themerc

@@ -9,6 +9,10 @@
 border.width: 1
 padding.height: 3
 
+# The following options has no default, but fallbacks back to
+# font-height + 2x padding.height if not set.
+# titlebar.height:
+
 # window border
 window.active.border.color: #dddad6
 window.inactive.border.color: #f6f5f4