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