theme: change padding.height default to 0

theme: change padding.height default to 0

...because now that window.button.height determines the height of button
hover effect the visible appearance of the titlebar will change unless
we reduce the padding to zero.

Backward compatibility notice: If a users theme sets padding.height to a
value greater than zero, the titlebar will be taller compared with
openbox. This can be fixed by either reducing window.button.height or
overriding padding.height