Set the default cursor theme to breeze_cursors

Summary: Currently the cursor theme is not set by anything, so it's Adwaita by default.

Test Plan: New user account has "Breeze" as cursor theme in kde-gtk-config.

Reviewers: Plasma, ngraham

Reviewed By: ngraham

Subscribers: ngraham, broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D17187