appIcons: Add support for translating plural forms

[3v1n0]

Closes: #2100

[xalt7x]

Thanks for the quick response.

Unfortunately it still doesn't work for me

1. make potfile still generates po/dashtodock.pot without plural forms

msgid "Quit %d Windows"
msgstr ""

However translation still works

2. I 've tried replacing "N__" with "ngettext"

this._quitMenuItem.label.set_text(N__(

this._quitMenuItem.label.set_text(ngettext(

After that po/dashtodock.pot receives plural strings

msgid "Quit %d Window"
msgid_plural "Quit %d Windows"
msgstr[0] ""
msgstr[1] ""

But after the actual build translation isn't used (strings remain in English)

[xalt7x]

Please review my PR to your branch which resolves the aforementioned issues.

[3v1n0]

Done your merge, let me know if this should go in now.

[xalt7x]

Works for me, thanks!