[tabbox] Correctly set global short on kwin tabbox

Summary:
We need to set both the current autoload shortcut and the default
shortcut. setGlobalShortcut is a convenience method for both.

Otherwise clicking defaults in the keys KCM will discard the shortcut
forever.

BUG: 407000

Test Plan:
wiped kglobalshortcutsrc to get a clean slate
restarted kwin
alt+tab
hit defauts in the kwin KCM
could still alt+tab

Reviewers: #kwin, zzag

Reviewed By: #kwin, zzag

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D24647