Use onClicked for Home button

Summary:
BUG: 407636
FIXED-IN: 5.16.0

Buttons should use onClicked instead of onPressed.

Test Plan: Click on the Home button. It now behaves correctly.

Reviewers: #plasma, #vdg, ngraham, apol

Reviewed By: #vdg, ngraham, apol

Subscribers: ngraham, #vdg, plasma-devel, #plasma

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21436