Setting a surface maximized if it is already in fullscreen mode

An attempt to provide some fixes for #321 (closed)

The first patch deals with it at the protocol level, avoiding to set the surface as maximized if already being set as fullscreen (according to protocol specs), and is the one that fixes the protocol violation we're seeing.

The second does the same thing, but at (client) shell level. In doing so, we follow other desktop environments which do not perform any action, until the user explicitly exits the fullscreen state.