theme: don't do harsh gradients for submenus

- use a more flat gradient for submenus. nothing is as curved
  and it doesn't help legibility.

https://bugzilla.gnome.org/show_bug.cgi?id=706037
theme: darken open submenu items

- avoid the clash of adjacent open and selected items

https://bugzilla.gnome.org/show_bug.cgi?id=706037
Merge branch 'menu-arrow'