effects/overview: Reuse qml loading results between effect activations

Currently, every time you launch the overview effect, QtQuick will go out and parse QML files. With the overview effect gaining more features and the code size getting bigger, it takes more time for the overview to present the first frame after it got triggered.

With this change, the overview effect will keep the ScreenView QQmlComponent object around that can be used to avoid reparsing qml code every time the overview effect is launched.

It also ports the Overview effect from de-facto deprecated qml context to initial properties, which yield slightly better startup times.