Port tooltips to KToolTipWidget

Summary:
KToolTipWidget (introduced by kwidgetsaddons 5.30) supersedes the
tooltip code (originally forked from Dolphin) in systemsettings.

Test Plan: Tooltips still work, both in X11 and Wayland.

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D4565