panelMenu: Use a normal St.Bin to allow styling

Currently we don't add some style to ButtonBox like padding etc, we add
it programatically taking into account minimum padding and natural
padding. The thing is that with natural padding it works as expected
even for low resolutions.

As a design request, we need to style from css, and since the current
ButtonBox class doesn't add any worth functionality, change that and use
a simple St.Bin that allow normal styling.