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.

`N_()` returns the string as it is ("untranslated" of course, as it does nothing), `_()` returns translated string if possible, depending on current language, in both cases passed string is the search key.
When regenerating `picard.pot`, a script looks for `_()` and `N_()` (and few others) to extract translatable strings.

So, sometimes you want to translate immediatly, ie. when you pass a string for immediate display, in this case, you should use `_()`. But when declaring strings in arrays, or dicts, most of the times you don't want immediate translation so you should use `N_()` to mark the string for translation (so it is extracted by i18n tools), and `_()` when displaying/printing/saving the translatable string. This is just the basic usage, check gettext documentation for more infos.