Use property for x11AppScreen in KWin::Application

Already provided by KWin::Application so let's use it instead of
QX11Info::appScreen.

REVIEW: 122683

libkwineffects/kwinglobals.h

@@ -170,7 +170,7 @@ KWIN_EXPORT xcb_screen_t *defaultScreen()
     if (s_screen) {
         return s_screen;
     }
-    int screen = QX11Info::appScreen();
+    int screen = qApp->property("x11ScreenNumber").toInt();
     for (xcb_screen_iterator_t it = xcb_setup_roots_iterator(xcb_get_setup(connection()));
             it.rem;
             --screen, xcb_screen_next(&it)) {

main.h

@@ -48,6 +48,7 @@ class KWIN_EXPORT Application : public  QApplication
     Q_PROPERTY(quint32 x11Time READ x11Time WRITE setX11Time)
     Q_PROPERTY(quint32 x11RootWindow READ x11RootWindow CONSTANT)
     Q_PROPERTY(void *x11Connection READ x11Connection NOTIFY x11ConnectionChanged)
+    Q_PROPERTY(int x11ScreenNumber READ x11ScreenNumber CONSTANT)
 public:
     /**
     * @brief This enum provides the various operation modes of KWin depending on the available