Add default shortcut to switch to the desktop to the left/right/top/bottom

Summary:
Very useful when you use virtual desktops. See T11520 for more information.

This reverts commit d72e96802b3c48ce69d8072cf30038d6ad552d0c.

Test Plan: Have 2 or more virtual desktops and use the new shortcut.

Reviewers: KWin, Plasma, VDG, romangg, ngraham, davidedmundson

Reviewed By: VDG, ngraham, davidedmundson

Subscribers: alexisd, alexde, ognarb, hpereiradacosta, broulik, davidedmundson, thiagosueto, ngraham, romangg, zzag, VDG, Plasma, kwin, KWin

Tags: KWin

Maniphest Tasks: T11520

Differential Revision: https://phabricator.kde.org/D24281