[ChangeLog][QtQuickDialogs] Added FileDialog. This is a native
FileDialog on platforms that support it, and a non-native
Qt Quick FileDialog on platforms that don't.

Fixes: QTBUG-87797
Change-Id: Ia3a98b616479b818c96c232a0329750023079642
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>