Adwaita GTK Theme: Add bigger shadow and border-radius to menus

I thought that the visibility of menus could be a little bit improved. Also most of the elements including the popopvers have a small border-radius, so I added a tiny border-radius for the menus, too.

To be concrete this is what this commit is about:

• Increase the visibility of the box-shadow for menus by decreasing the transparentize and increasing the blur-radius
• Introduce a border-radius variable for menus
• Use this variable for all corners of menus except top for the top menus -> Exclude the top right and left border-radius of menus populated out of menu-bars so the edge of the menu is still completely attached to the menubar
• Increase the padding of the menus, so the first and last child of the menu is not cut by the border-radius

Before:

After: