Add KIO::OpenUrlJob::setShowOpenWithDialog as replacement for KRun::displayOpenWithDialog

Summary:
This will not look up the preferred app or run scripts/executables, it always shows an open-with dialog.
Bonus feature compared to KRun::displayOpenWithDialog or KOpenWithDialog: it will first determine the mimetype
(even for remote URLs), which allows to "remember this app for this mimetype".

On the other hand, this loses the support for multiple URLs; OpenUrlJob is about a single URL.
I guess for multiple URLs the best solution is to use KOpenWithDialog directly.

Test Plan: New unittest

Reviewers: ahmadsamir, broulik, svuorela

Reviewed By: ahmadsamir

Subscribers: kossebau, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D29558