Revert "applet: remove unused QtQuick.Controls import"

This reverts commit 2fb607aa.

In fact it was used, but this went unnoticed because it wasn't
namespaced, so the thing using it wasn't obvious.

plasma/package/contents/ui/main.qml

@@ -5,6 +5,7 @@
  */
 
 import QtQuick 2.4
+import QtQuick.Controls 2.4
 import QtQuick.Layouts 1.1
 import org.kde.plasma.core 2.0 as PlasmaCore
 import org.kde.plasma.plasmoid 2.0