[ExpandableListItem] make it touch-friendly

Summary:
Right now ExpandableListItem is not very touch-friendly due to use of a mousearea
hover effect for choosing the selected item. This patch fixes that by using a TapHandler
to handle left-clicks and touch input. We still need a MouseArea inside it to handle
non-touch hover and right-click behaviors.

FEATURE: 393749
FIXED-IN: 5.70

Test Plan: Click behavior is identical to how it was before, but now touch works too:

Reviewers: Plasma, apol, VDG, niccolove

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28808