[kcmkwin/compositing] Accept focus on the EffectView

This allows keyboard navigation with cursor keys in the list view.

CCBUG: 335195

kcmkwin/kwincompositing/qml/EffectView.qml

@@ -168,6 +168,7 @@ Item {
                 section.property: "CategoryRole"
                 section.delegate: sectionHeading
                 spacing: col.spacing
+                focus: true
             }
         }