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

Summary: Same as on Windows 10, very useful when you use virtual desktops.

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

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

Reviewed By: KWin, Plasma, VDG, ngraham, davidedmundson

Subscribers: davidedmundson, thiagosueto, ngraham, romangg, zzag, VDG, Plasma, kwin, KWin

Tags: KWin

Maniphest Tasks: T11520

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