Add localisation for Keybind modifiers

[dengelt]

Currently, the Display trait of the KeyBind type just outputs the English text for the key bind. However, other languages have different names for keys and modifiers (e.g. in German "Del" -> "Entf", "Ctrl" -> "Strg"). libcosmic doesn't do i18n at the moment, but it seems there is a PR #556 that adds i18n support, though not yet for key binds.

I18n support for key binds would be neccessary for pop-os/cosmic-files#314