[kcmkwin/compositing] Accept StrongFocus on the QuickView's WidgetContainer

This ensures that we can tab to the UI written in QtQuick. Unfortunately
it doesn't get initial focus, which seems to be a general problem of
KCMShell (QWidget based KCMs also do not get focus) and it's not possible
to tab out of quick ui again.

BUG: 335195

kcmkwin/kwincompositing/main.cpp

@@ -82,6 +82,7 @@ KWinCompositingKCM::KWinCompositingKCM(QWidget* parent, const QVariantList& args
     connect(m_view.data(), &KWin::Compositing::EffectView::changed, [this]{
         emit changed(true);
     });
+    w->setFocusPolicy(Qt::StrongFocus);
 }
 
 KWinCompositingKCM::~KWinCompositingKCM()