Summary:
use kactivities to track the most used modules (kcmshell will need to as well)
and present them in the start page as 5 big icons

Test Plan:
empty kactivities db, show default set, after some usage, the most used modules come
on top

Reviewers: #plasma, #vdg

Subscribers: davidedmundson, abetts, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6061