[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