[kcmkwin] Apply grid delegate KCMs on double-click

To be consistent with plasma-workspace KCMs which we applied this to in
https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/606

src/kcmkwin/kwindecoration/package/contents/ui/Themes.qml

@@ -103,6 +103,9 @@ KCM.GridView {
             kcm.theme = index
             view.currentIndex = index
         }
+        onDoubleClicked: {
+            kcm.save();
+        }
     }
     Connections {
         target: kcm

src/kcmkwin/kwinvirtualkeyboard/package/contents/ui/main.qml

@@ -35,5 +35,8 @@ KCM.GridViewKCM {
         onClicked: {
             kcm.settings.inputMethod = model.desktopFileName;
         }
+        onDoubleClicked: {
+            kcm.save();
+        }
     }
 }