Traverse the stacking order backwards when switching between windows

Summary:
In order to pick the next client to activate we traverse the stacking
order from bottom to top and assign to each client a score. The client
with the best score will be activated next. Function that assigns score
bases its decisions purely on geometry. This may backfire if there are
couple maximized or fullscreen clients on the screen - we'll activate
the bottom-most client.

This change toggles direction we traverse the stacking order. If there
are several clients with an identical score, then prefer the top-most
client, the one that the user most likely sees at the moment.

BUG: 411356
FIXED-IN: 5.17.0

Test Plan: New tests pass.

Reviewers: KWin

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23521