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

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.