Add KIO::DropJobFlag to allow manually showing the menu

Summary:
This patch allows the application to manually popup the menu by using the KIO::ShowMenuManually flag with the DropJob::showMenu() method.

BUG: 415917

Test Plan: Can't reproduce the bug with this patch.

Reviewers: Frameworks, davidedmundson, elvisangelaccio, mart, dfaure

Reviewed By: dfaure

Subscribers: ngraham, broulik, anthonyfieroni, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D26484