KUndoActions: use actionCollection to set the shortcut

Summary:
Resolve the following warning printed out when the undo/redo actions
are added through KUndoActions:

KXMLGUIFactoryPrivate::saveDefaultActionProperties: Shortcut for
action "edit_undo" "Undo" set with QAction::setShortcut()!
Use KActionCollection::setDefaultShortcut(s) instead.

Test Plan: The undo/redo actions still work and the warning is gone.

Reviewers: aacid

Reviewed By: aacid

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7195