[kcmkwin/kwindesktop] Fix global shortcut's display name for kwin changing

Summary:
KGlobalAcceld stores actions grouped by ID and a single display name.
When an action with a given id changes, the display names change for all
actions in that
group.

The KCM sets the ID to "kwin" but does not explicitly set a display
name. This means it is automatically deduced which will be kcmshell or
systemsettings.

The end result is all kwin shortcuts get renamed in system setting's
global shortcuts UI until kwin is restarted.

This patch explicitly sets the dispay name.

BUG: 400248
Fixed-in: 5.15.0

Test Plan:
Changed virtual desktops
Checked global shortcut settings still listed kwin as kwin

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: graesslin, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D16418