theme: add window.titlebar.padding.{width,height}

...to replace padding.{width,height} to minimize breaking changes with the
visual appearance of the titlebar.

With the diverging labwc specification for the titlebar (listed below)
we have to choose between (a) not supporting the padding.{width,height}
option which exist in many extant Openbox themes to keep titlebar height
(almost) the same; or (b) making the allocated button areas much smaller
and not keeping the default hover going all the way to the edges. All in
all it just seems a lot simpler and cleaner to break this link to the
openbox spec.

Examples of previous change driving the requirement for this change:

- SVG and PNG support which often results in large icons with hover
  effects.
- Theme option window.button.{height,width}
- Larger default areas for icons (26x26)

In way of an example, Numix theme sets a padding.height of 6 which would
have resulted in a titlebar 12px taller without this change.

docs/labwc-theme.5.scd

@@ -50,14 +50,13 @@ labwc-config(5).
 	Line width (integer) of border drawn around window frames.
 	Default is 1.
 
-*padding.width*
-	Horizontal padding size, in pixels, between border and first
+*window.titlebar.padding.width*
+	Horizontal titlebar padding size, in pixels, between border and first
 	button on the left/right.
 	Default is 0.
 
-*padding.height*
-	Vertical padding size, in pixels, used for spacing out elements
-	in the window decorations.
+*window.titlebar.padding.height*
+	Vertical titlebar padding size, in pixels.
 	Default is 0.
 
 *menu.items.padding.x*