Varible session_state moved to global namespace

The variable session_state, defined in screen.h (WScreen), is moved to
the global variable w_global.

Signed-off-by: Rodolfo García Peñas (kix) <kix@kix.es>

src/screen.h

@@ -240,8 +240,6 @@ typedef struct _WScreen {
     /* state and other informations */
     short cascade_index;	       /* for cascade window placement */
 
-    WMPropList *session_state;
-
     /* for double-click detection */
     Time last_click_time;
     Window last_click_window;