Added a menu option to add 'new' files to tagger

A file is considered 'new' if it has been modified in the last five hours. This setting can be stored in the config later. Only the starting directory is scanned for new files currently. A context menu option can be added to the treeview to add new files from a folder.

I would probably not set a time here but e.g. `None` and skip the whole filtering in the `get_files` function above if `period == None`. That saves all the `getctime` calls in this common case.