Summary:
This patch changes the icons in the settings' window's sidebar to be all colorful, in
conformance with T10165 specifically, and the new #consistency goal more generally.

It also cleans up the code a tiny bit by using built-in constructor functionality to set
icons from the icon theme rather than leaving the icon blank in the constructor and then
setting it later.

Test Plan:
Before: {F7449955}
After: {F7449956}

Reviewers: #vdg, #kde_partition_manager, stikonas, ndavis

Reviewed By: #vdg, #kde_partition_manager, stikonas, ndavis

Maniphest Tasks: T10165

Differential Revision: https://phabricator.kde.org/D24272