[Task Manager] Sort manually by default

Summary:
This patch changes the default Task Manager sort order to Manually, which better matches
today's user expectations regarding how tab bars should work, and this is basically a systemwide tab bar.

Test Plan: New default took effect for new user account and existing user account where I hadn't manually changed the sort order

Reviewers: VDG, Plasma, hein, ndavis, mart

Reviewed By: VDG, Plasma, hein, ndavis, mart

Subscribers: ndavis, alexeymin, broulik, mart, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D18651