[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