Avoid double-shadow around message window

The shadows of two containers around the message window look a bit strange. With a 3D theme it looks the message window is too deep, with a 2D theme there is a thick line around the message window. I think the message window should be at the same "level" as the rest of the main window components - the sidebar and the editor - which don't use any shadow like that. This patch removes the shadows (edited manually, the diff made by glade was too big).

I've been looking at this for about 4 years now - I have finally managed to overcome my laziness :-)