Flutter_i18n fixes

Closes #145 (closed)

• Identified issue in flutter_i18n (null safety in simple_translator.dart)
• upstream library fixes see this issue in library repo. The maintainer can't reproduce our bug and therefore closed the ticket.
• Added missingTranslationHandler to FlutterI18nDelegate call
• Flattened FileTranslationLoader call in main.dart
• Made JsonDecodeStrategy() the default decoding strategy
• Refactored the translation code a bit
• Moved supportedLocales and decodeStrategies Lists to localization_constants.dart and added masking to the import statements in the files using localization_constants.dart thus the suggestions by the IDE while coding becomes a bit less clobbered. (Might be favorable for future files containing constants etc.)
• Added an exception as well as some code for handling missing translation in the future. (just has to be incommented back in once there is a new compatible i18n version)
• bump flutter_i18n dependency to version v0.16.x after library has been fixed see above. Reverted to v0.14.0 as it provides the best compatability so far. Hopefully a future version will somehow fix this.