Add translation keys for all user-facing strings

[introkun]

Initial translation was introduced by #228. We need to replace all hard-coded user-facing strings with translation keys (e.g. 'ui_component.sub_component.title').

For example, we can replace the hard-coded string (Scan) with key (main.top_menu.scan): https://github.com/blawar/nut/commit/bb5ba87692c7d08558a07ee55ff0006f1871c387#diff-042518e80c2b33186269a1e46b893f6943e77a8da8fb361307801ae27d7a72a6R66 And add English translation to the translate.json (section None is for English strings):

...,

"main.top_menu.scan": "Scan"

...

[eXhumer]

Already merged with #228.

[eXhumer]

Looks like that commit didn't completely replace all user facing strings with translation keys.

[introkun]

Added few more in #267