QXdgDesktopPortalFileDialog: use QUrl to store directory & selection

Fixes QXdgDesktopPortalFileDialog::directory() returning a broken QUrl.
while making other code consistent & more fail-proof.

Old code had some mixups where the used plain strings had file:/// scheme
prefixes if read out from the "uris" of the D-Bus replies, but not when
generated from QUrls passed in by the own API.
Also is the usage of QUrl::fromLocalFile/toLocalFile preferred over
getting a QUrl's  path() property.