The return of URI-specific context menus

They were lost when Gajim switched to MessageLabel for message representation (on most platforms).

I went ahead and refactored the elifen switch into a data structure that's checked for completeness against URIType and XmppUriQueryType enums. The check is not completely automated: it's a simple assertion at module level, so it's triggered at load time, so it might be a good idea to add some tests that merely import modules.

All URITypes are now covered, and some more menu items are added here and there. Some items' labels get an ellipsis appended outside the translatable strings so as not to invalidate existing translations, but I'm not sure it is worth the strange look of the code. Should I simply change the strings?