Allow to drop one file or one folder on KDirOperator

Summary:
BUG: 45154
FIXED-IN: 5.59

Test Plan:
* Manual testing using kfilewidgettestgui.cpp and kfilewidgettest_saving_gui.cpp
* added automated test

Reviewers: ngraham, #frameworks

Reviewed By: ngraham

Subscribers: elvisangelaccio, apol, kde-frameworks-devel

Tags: #frameworks

Maniphest Tasks: T8552

Differential Revision: https://phabricator.kde.org/D20838