Use large colorful category icons in Dolphin settings

Summary:
{F7324074}
Ref task T10165
Depends on D23749

Test Plan: Open the Dolphin settings dialog.

Reviewers: #dolphin, #vdg, ndavis, ngraham

Reviewed By: #dolphin, #vdg, ndavis, ngraham

Subscribers: ngraham, ndavis, kfm-devel, #vdg, #dolphin

Tags: #dolphin

Maniphest Tasks: T10165

Differential Revision: https://phabricator.kde.org/D23750