Use asynchronous creation for several things when activating QtQuick scene effects

Currently, there is a noticeable delay when activating the window view effect from Plasma's taskbaar. This is because there's a bunch of time spent creating views for the QML scene. This MR changes the way views are created and several other pieces so we use asynchronous creation rather than blocking until everything is created. This allows KWin to perform other things while the views are created and removes practically most of the activation lag.