kcms/decoration: Fix highlight when hovering the preview
The preview item implements hover event filters, which by default are accepted and not propagated to their parent, breaking the hightlight on hover for the GridView.
We can simply "ignore()" those events so they are propagated, but still fully functional.
Preemptively, do the same also for mouse events, which seem to be already propagated now, but shouldn't according to the QQuickItem documentation.