kwineffects: Make OffscreenQuickView generate double click events

Currently, MouseArea.doubleClicked doesn't work in QtQuick effects. Luckily, the overview effect doesn't use it, but it's still worth making sure that it works as expected regardless of whether it's a kwin effect.

With this change, the OffscreenQuickView will keep track of the last mouse press event. If the second button press occurs within the mouse double click interval, the OffscreenQuickView will generate a QEvent::MouseButtonDblClick event, similar to what QGuiApplication would normally do when processing window system events.