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: {F6515354, size=full}

Reviewers: #vdg, #plasma, mart, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

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