Make WindowSwitching (Alt+Tab) the default left touch screen edge

Summary:
Based on the feedback from discussion on plasma-devel the currently only
default touch action on a screen edge should be window switching. Given
that our default window switcher is located on the left and works very
nicely for touch event it becomes the default action on the left screen.

Reviewers: KWin, Plasma

Subscribers: plasma-devel, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D5551