use the standard system download path for downloads

QFileDialog's dir arg (which was set to the incoming file name from the
Matrix download) can take a full path to suggest. By prepending
QStandardPaths::DownloadLocation, it opens to the system's download
folder and proposes the filename as the download name.

Using QStandardPaths should make this work on other platforms, and from
what I read, its possible for this to return an empty string on
platforms where it doesn't support it, so this should essentially
revert to the previous functionality if Qt can't determine the system's
download location.