client: menu: make RedWindow::set_menu() return an error-code (#758260)

RedWindow::set_menu() can fail (on Windows when in fullscreen mode).
For Windows spice-client, when in fullscreen mode, the system-menu
is NULL.

Returns 0 upon success, non-0 (currently only -1) upon failure.

(cherry picked from commit 24d5852611)
(seperator vs separator --> a small typo that got fixed)

client/red_window.h

@@ -70,7 +70,7 @@ public:
     void release_mouse();
     void start_key_interception();
     void stop_key_interception();
-    void set_menu(Menu* menu);
+    int set_menu(Menu* menu);
 
 #ifdef USE_OGL
     void untouch_context();

client/windows/red_window.cpp

@@ -1066,18 +1066,26 @@ void RedWindow_p::release_menu(Menu* menu)
     }
 }
 
-void RedWindow::set_menu(Menu* menu)
+int RedWindow::set_menu(Menu* menu)
 {
     release_menu(_menu);
     _menu = NULL;
 
     if (!menu) {
-        return;
+        return 0;
     }
-    _menu = menu->ref();
+
     _sys_menu = GetSystemMenu(_win, FALSE);
+    if (! _sys_menu) {
+        return -1;
+    }
+
+    _menu = menu->ref();
+
     insert_seperator(_sys_menu);
     insert_menu(_menu, _sys_menu, _commands_map);
+
+    return 0;
 }
 
 static LRESULT CALLBACK MessageFilterProc(int nCode, WPARAM wParam, LPARAM lParam)

client/x11/red_window.cpp

@@ -2215,8 +2215,9 @@ void RedWindow::on_pointer_leave()
     }
 }
 
-void RedWindow::set_menu(Menu* menu)
+int RedWindow::set_menu(Menu* menu)
 {
+    return 0;
 }
 
 void RedWindow::init()