New shutdown.h file

The new shutdown.h file includes the shutdown modes and the functions
to shutdown Window Maker.

The function definitios were moved from funcs.h and the struct WShutdownMode
from WindowMaker.h.

Now, externs are not needed, only include shutdown.h

src/WindowMaker.h

@@ -87,16 +87,6 @@ typedef struct WObjDescriptor {
     void *parent;		       /* parent object (WWindow or WMenu) */
 } WObjDescriptor;
 
-
-/* shutdown modes */
-typedef enum {
-    WSExitMode,
-    WSLogoutMode,
-    WSKillMode,
-    WSRestartPreparationMode
-} WShutdownMode;
-
-
 /* internal buttons */
 #define WBUT_CLOSE              0
 #define WBUT_BROKENCLOSE        1