[PrinterSortProxyModel] Make filter case-insensitive

Summary:
This patch slightly improves usability of the printers plasmoid:
everywhere in the Plasma, filters / searches are case-insensitive.
Make it so for printers applet as well.

Test Plan: It compiles, it works

Reviewers: ngraham

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D21962