Fix W32 theme for spinbuttons

Fix W32 theme for spinbuttons

It seems that the dec-button no longer has a "bottom" selector-thingy to
select for. Use "last-child" instead.
Use .vertical to style vertical instance differently, as last-child/first-child
meaning is inverted there.
Increase (towards 0) margins a bit, to avoid clipping the pre-light rectangle.
Use dir(rtl) selector to style horizontal instance differently for RTL locales,
where first-child is "inc".

https://bugzilla.gnome.org/show_bug.cgi?id=727022