Summary:
when searching, show categories *and* subcategories indented
under categories, making it way less confusing

when the user clicks at a search result the proper
category and subcategory will open

Also when clicking on most used, the proper
category/subcategory will get open on the
sidebar

BUG: 400303
BUG: 398619
FIXED-IN: 5.18.0

Test Plan:
{F7819885}
Also old views icons and tree still work as usual

Reviewers: #plasma, #vdg, davidedmundson

Reviewed By: #plasma, #vdg, davidedmundson

Subscribers: ngraham, davidedmundson, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26021