add tooltips to the intro page

Summary:
Add tooltips to the most used settings in the intro page

Test Plan: Move your cursor on one of the most used settings, a tooltip will appear

Reviewers: VDG, ngraham, GB_2

Reviewed By: VDG, ngraham

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D24049