Don't accept hover events at the bottom of ApplicationItem

It breaks some buttons in a subtle way. For example, in KScreen KCM, single refresh rate label's contextual help button won't get a hover state due to this notification manager blocking events from reaching it.

Setting this property to false won't propagate in the way it does with visibility: only those controls which don't have an explicit hoverEnabled set will be affected. Normally, controls which can deal with hover events do set it ON manually.

KCM_context_info_button_hover_broken_by_Kirigami

CC @sporknife