Wrapping user-facing strings in NSLocalizedString

[BasThomas]

This pull request wraps user-facing strings in NSLocalizedString, so that they could be localized at a later point in time. Addresses #141.

To note:

• Some strings (particularly, the lockedUntil ones) suggest they may be used as identifiers; verify those do not break (and consider refactoring them to use unlocalized identifiers instead).
• Same for the currently enabled theme.
• Actual localization (.strings file, actual localizations in another language) are not tackled by this pull request.
• We may want to consider unifying capitalization in words, and changing case using .uppercased() so duplication based on case is not required.
• We may want to consider looking into .stringsdict for pluralization rather than "duplicating" the strings.
• This pull request does not add comments for localized strings.

[twostraws]

Hi Bas! Thanks for this. I'm a bit worried about the strings you mentioned that might affect functionality – this isn't a change I can easily merge in unless it gets some further verification.