Reverse the meaning of BWindow fShowLevel to match BView.

This also matches the client_window_info.show_hide_level field used in Deskbar
and other applications.

While doing this, keep fShowLevel fully in sync between BWindow and app_server,
use one message type for both hiding and showing, and make the decision to show
and hide the window in the app_server.

Lastly make minimize behave as described in the Be Book: hidden windows cannot
be minimized, and minimized windows which get hidden become unminimized.

headers/os/interface/Window.h

@@ -352,6 +352,7 @@ private:
 
 			void				_GetDecoratorSize(float* _borderWidth,
 									float* _tabHeight) const;
+			void				_SendShowOrHideMessage();
 
 private:
 			char*				fTitle;

headers/private/app/ServerProtocol.h

@@ -87,8 +87,8 @@ enum {
 	AS_GET_CURSOR_BITMAP,
 
 	// Window definitions
-	AS_SHOW_WINDOW,
-	AS_HIDE_WINDOW,
+	AS_SHOW_OR_HIDE_WINDOW,
+	AS_INTERNAL_HIDE_WINDOW,
 	AS_MINIMIZE_WINDOW,
 	AS_QUIT_WINDOW,
 	AS_SEND_BEHIND,