Add Russian translations for the desktop entries and AppStream metadata

[wehrwolfmann]

Protontricks has no translations at the moment, so a Russian user sees the English name and description in the application menu and in the software centre.

This adds Russian to the places that are translated declaratively and need no new build machinery:

• protontricks.desktop: GenericName[ru], Comment[ru], Keywords[ru]
• protontricks-launch.desktop: Name[ru], Comment[ru]
• the AppStream metainfo: xml:lang="ru" for the summary, the description and the screenshot caption

Two English keys are added along the way, since a localised key without its untranslated counterpart looks odd: GenericName for the main entry, and Comment for the launcher entry, which had no description at all.

Checked with desktop-file-validate (both files pass) and appstreamcli validate --pedantic (passes; the only remaining hint is the pre-existing uppercase letter in the component ID). Verified with GLib.KeyFile.get_locale_string that both entries resolve to the Russian strings under ru_RU.UTF-8 and are unchanged under other locales.

No gettext machinery is proposed here. Most of the remaining user-visible text is command-line diagnostics that people paste into bug reports, and wiring up message catalogues for it would be a much bigger change than it is worth. If you would like proper i18n later, the dialog strings in gui.py are the part that would actually benefit.

[Matoking]

Thanks, I've merged the changes.