Show tooltips for subcategories

Summary:
Tooltips are already shown for both categories as well as top-level items
at the category level (e.g. Printers). This patch makes the tooltips
show up for subcategory items as well.

FEATURE: 402651
FIXED-IN: 5.15.0

Test Plan:

Reviewers: VDG, Plasma, mart, davidedmundson

Reviewed By: Plasma, davidedmundson

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17864