Add proper i18n

[deobald]

Currently, strings are looked up in https://github.com/pariyatti/patta/blob/master/lib/resources/strings.dart

Instead, we should use one of the available internationalization (i18n for short) libraries out there. Before we add one, though, we should review what's available:

• Official approach: https://flutter.dev/docs/development/accessibility-and-localization/internationalization
• JSON lookups: https://pub.dev/packages/flutter_i18n
• YAML lookups: https://pub.dev/packages/i18n

Steps:

• [x] Evaluate all three (and possibly other options!)
• [x] Make an argument for the use of one particular library based on its maturity, simplicity, features, and future sustainability/longevity
• [x] Finally, integrate it into the app to replace strings.dart.

[deobald]

The solution chosen in #132 is really not far from the original strings.dart approach... it's just a basic lookup table with fall-through to English. The standard Google i18n library is a bit too heavy for us right now, and we can look at switching to it if/when we have a real Flutter dev on the team.