popupMenu: Rename _arrow_rotation to follow JS covention
popupMenu: Use relative rotation for arrows

Currently the animation of the arrows doesn't take
into account previous rotation.
Since in a incoming patch we will use one arrow
and rotate it to generate the four directions, we
need that the animation use relative rotation.

https://bugzilla.gnome.org/show_bug.cgi?id=720206
popupMenu: Use an image for arrows

Until now the arrows were the associated arrow
character of the font. This cause some problems like
different arrows for different fonts, and size can be
altered because of the font size.
To solve that, use an image for the arrows.

https://bugzilla.gnome.org/show_bug.cgi?id=720206