Prefer getSelectedFiles to getFiles

Anyway, add extension dialog allows to select only 1 file.
So getFiles and getSelectedFiles both return a 1 cell array which contain the full path of the file.

See http://nabble.documentfoundation.org/Multiselection-needs-work-tt4153207.html

Change-Id: I475ea519648d075b482a07d23b15af2ddb8f4637
Reviewed-on: https://gerrit.libreoffice.org/18711Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Julien Nabet <serval2412@yahoo.fr>

desktop/source/deployment/gui/dp_gui_dialog2.cxx

@@ -936,7 +936,7 @@ uno::Sequence< OUString > ExtMgrDialog::raiseAddPicker()
         return uno::Sequence<OUString>(); // cancelled
 
     m_sLastFolderURL = xFilePicker->getDisplayDirectory();
-    uno::Sequence< OUString > files( xFilePicker->getFiles() );
+    uno::Sequence< OUString > files( xFilePicker->getSelectedFiles() );
     OSL_ASSERT( files.getLength() > 0 );
     return files;
 }