Correctly extend context menu of file view

Summary:
Overriding KDirOperator::activatedMenu() leads to incomplete
initialization of context menu, in particular "Create New"
menu remain empty. Extend context menu on
KDirOperator::contextMenuAboutToShow() signal.

Reviewers: #kde_applications, aacid

Reviewed By: #kde_applications, aacid

Differential Revision: https://phabricator.kde.org/D18184