FilePopupMenu: pluralize selected files

[slook]

• Added: "1 File Selected" or "n Files Selected" translatable strings for the file popup menus
• Removed: "n File(s) Selected" translatable string

The idea of including the numeric digit within singular version of the string is to help the translators. It also allows flexibility if a number is not desired, such as "No Files" or just the word "Folder" without a number, for example.

[mathiascode]

Apparently gettext.install() can also make ngettext() available, so you can gettext.install(names=["ngettext"]) in i18n.py, then do something like:

        self.items["selected_files"].set_label(
            ngettext("{num} File Selected", "{num} Files Selected", num_files).format(num=num_files)
        )

[slook]

It would be nicer to wrap the suggested ngettext() implementation into a function which only requires the key (e.g. "selected_files") to look-up the assigned strings. Otherwise it will be too cumbersome to use when defining menu items and labels etc.

This would also give the intended advantage of having strings defined in the core to be used also in headless mode.

[mathiascode]

It depends. Ideally we would only use a string once in the codebase anyway. We could probably start by defining these strings in popupmenu.py only, and move them somewhere else if they're needed elsewhere in the future.