dive download: pass current filename to dialog

Instead of accessing a global variable, pass the filename
from the MainWindow to the dialog. This is supposed to cut
down on the global variable mess.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

desktop-widgets/mainwindow.cpp

@@ -607,7 +607,8 @@ void MainWindow::on_actionQuit_triggered()
 
 void MainWindow::on_actionDownloadDC_triggered()
 {
-	DownloadFromDCWidget dlg(this);
+	QString filename = existing_filename ?: prefs.default_filename;
+	DownloadFromDCWidget dlg(filename, this);
 	dlg.exec();
 }