[Cuttleffish] Use proper QSortFilterProxyModel

Summary:
Don't reload everything when filter or category is changed. Fixes selected icon
changing when filtering and the first icon not being selectd when the current
icon is selected.

Test Plan: select and filter

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: apol, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D27421