Add support for importing of metadata
RDFTreeCtrl: Handle model change events
MenuCtrl::askForFile: accept extensions filters
MenuCtrl::importMetaData: set ExtensionsFilters
MenuCtrl: use Opt.ofNullalbe for chooser results
MenuCtrl: askForFile takes a title param now

Make it more generic
MenuView: move "Import Metadata" to project menu

Also add trailing "..." to indicate it will open a dialog.
Merge pull request #49 from gicmo/md_import

Curde support for metadata import

LGTM