The arrow is part of the background image drawn by the theme, don't draw
GTK's own arrow. This also applies to non-entry comboboxes.
On XP the arrow-and-nothing-else part does not exist (W32 XP theme draws a
white rectangle) and has to be subtituted for a simple dropdown button even
for non-entry comboboxes.
https://bugzilla.gnome.org/show_bug.cgi?id=727035
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
