GTK L&F: menu separators do not match native ones

See attached screenshots (terminal-submenu-native.png shows native GTK menu,
terminal-submenu-java.png shows Swing GTK L&F from Mustang b63 on Solaris Nevada b27a).
Note the differences with the Blueprint theme (similar issues with other themes):
  - native separator images is 3 pixels tall, Swing's is only 2 pixels tall
  - native separator has some padding on top and bottom, Swing's does not
  - native separator has padding on left and right (about 5 pixels on each side),
    Swing's extends all the way to the edges