gtk3_kde5: Make folder selection more intuitive

When 'QFileDialog::Directory' is set, the native
Plasma QFileDialog does not consider the clicked
directory to be selected, but rather the base directory
shown in the dialog, s.
https://bugs.kde.org/show_bug.cgi?id=406464 .

Therefore don't set the option when in a KDE Plasma desktop
environment (which the kde5 VCL plugin also doesn't do).

This works around the above issue in the Plasma desktop
integration.

Change-Id: Ib3d0978ab56b6e50ee45f7ad997ec051b35faf54
Reviewed-on: https://gerrit.libreoffice.org/71223
Tested-by: Jenkins
Reviewed-by: Michael Weghorn <m.weghorn@posteo.de>
(cherry picked from commit 9fc0dbcd)
Reviewed-on: https://gerrit.libreoffice.org/71274Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>

vcl/unx/gtk3_kde5/kde5_filepicker.cxx

@@ -65,7 +65,16 @@ KDE5FilePicker::KDE5FilePicker(QObject* parent)
 void KDE5FilePicker::enableFolderMode()
 {
     _dialog->setOption(QFileDialog::ShowDirsOnly, true);
-    _dialog->setFileMode(QFileDialog::Directory);
+    // Workaround for https://bugs.kde.org/show_bug.cgi?id=406464 :
+    // Don't set file mode to QFileDialog::Directory when native KDE Plasma 5
+    // file dialog is used, since clicking on directory "bar" inside directory "foo"
+    // and then confirming would return "foo" rather than "foo/bar";
+    // on the other hand, non-native file dialog needs 'QFileDialog::Directory'
+    // and doesn't allow folder selection otherwise
+    if (Application::GetDesktopEnvironment() != "KDE5")
+    {
+        _dialog->setFileMode(QFileDialog::Directory);
+    }
 }
 
 KDE5FilePicker::~KDE5FilePicker()