Set transientParent window for file dialog

The file dialog did not set a proper WM_TRANSIENT_FOR hint. This change
ensures that the proper parent window is passed as the WM_TRANSIENT_FOR
hint.

BUG: 367699
FIXED-IN: 5.7.5

Test Plan: xprop+xwininfo on file dialog and parent window

Reviewers: Plasma

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2543