Use Breeze color scheme by default

Summary:
Various people have suggested over the years that we use the Breeze
color scheme by default, to improve continuity with other KDE
applications. This patch makes it happen.

BUG: 391920
FIXED-IN: 19.04.0

Test Plan:
Create new user account and open Konsole:
{F6485803, size=full}

Reviewers: #konsole, #vdg, hindenburg

Reviewed By: #konsole, hindenburg

Subscribers: konsole-devel

Tags: #konsole

Differential Revision: https://phabricator.kde.org/D17648