[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