[Task Manager] Use ToolButtons for buttons in popup, rather than mouseArea+icon

Summary:
Right now, the Task Manager's popup has various buttons in it that do not look or
feel like buttons because they are implemented as a MouseArea with an Icon inside.

This patch makes them feel like standard buttons uses elsewhere by implementing
them using ToolButtons instead.

Test Plan:
{F6874640, size=full}
{F6874639, size=full}

Reviewers: #plasma, #vdg, ndavis, filipf, broulik

Reviewed By: #plasma, #vdg, ndavis, filipf, broulik

Subscribers: broulik, abetts, filipf, ndavis, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D21629