Description of Problem:
Setting the background type to transparent for the panel(gnome-panel) should
make the menus transparent as well, otherwise it's inconsistent.

Steps to reproduce the problem:
1. Right-click on a panel with a menu included
2. Select properties
3. Switch to the background tab
4. In the type chooser, select transparent
5. Click close
6. Click on the menu icon
7. See how the menu coming out from it transparent parent has a solid background
color

Actual Results:
The menu is filled (full opacity)

Expected Results:
If the menu is originating from a transparent panel, one would assume that the
menu should be transparent as well.

How often does this happen?
Always

Additional Information:
There could be an option to *not* make the menu follow the panel appearance, but
the default behaviour should still be that the menu follows that of the panel.