Correct usage of KActionCollection

Summary:
Fixes this warning: "Shortcut for action
"katekonsole_tools_toggle_focus" "&Focus Terminal" set with
QAction::setShortcut()! Use KActionCollection::setDefaultShortcut(s)
instead."

Reviewers: #kate, cullmann

Reviewed By: #kate, cullmann

Subscribers: kwrite-devel

Tags: #kate

Differential Revision: https://phabricator.kde.org/D16821