kcms/cursortheme: Fix highlight when hovering the preview
The PreviewItem 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.
Similar to !3195 (merged) and kwin!4335 (merged).
Edited by Ismael Asensio