Fix missing setDefaultShortcuts call

Summary:
The commit eec49bc38f4e256b66bf16ee5428c5f5d7e97e25 cased a warning to
be emitted when starting Dolphin:

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

The fix is to call
actionCollection()->setDefaultShortcuts(m_forwardAction, m_forwardAction->shortcuts());
so that the default shortcuts are setup.

Reviewers: elvisangelaccio

Reviewed By: elvisangelaccio

Subscribers: kfm-devel

Tags: Dolphin

Differential Revision: https://phabricator.kde.org/D24342