tdf#121276 gtk3_kde5: Don't prevent creating new files

This takes over for gtk3_kde5 what was done for kde5 in commit
c0f45c89.

Change-Id: Ieca71498bd0cc00ca07b83f6e1178d983169d5be
Reviewed-on: https://gerrit.libreoffice.org/63172
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>

vcl/unx/gtk3_kde5/kde5_filepicker.cxx

@@ -88,6 +88,9 @@ bool KDE5FilePicker::execute()
 
 void KDE5FilePicker::setMultiSelectionMode(bool multiSelect)
 {
+    if (_dialog->acceptMode() == QFileDialog::AcceptSave)
+        return;
+
     _dialog->setFileMode(multiSelect ? QFileDialog::ExistingFiles : QFileDialog::ExistingFile);
 }