[applets/appmenu] Use libtaskmanager for appmenus

Summary:
This patch ports the appmenu applet to use
libtaskmanager.

Test Plan: Ensure that there aren't any regressions.

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28146