There is now a code path to render a generic QtQuick window via the
effects framework. We can use this as the implementation for the
existing EffectFrame API.
This allows us to get rid of a /lot/ of custom rendering that tangles
it's way through the backend, as well as getting rid of lowlevel usage
of Plasma's FrameSVG classes which may not exist in KF6, using the much
more optimised FrameSVGItem.
Cross fade is now done automatically by IconItem.
EffectFrame::setSelection is deprecated as I had no idea what it was
for, and no-one used it.
- I need a slight mod to hook up the path that allows effects to affect frames.
Not quite sure what level we want to do that.
- Even though this new code is super simple, retrofitting to match the
old API correctly is somewhat challenging. IMHO we should just break things a bit.