Also:
- provide a way for the progress handler to skip the throttling so that
we can ensure progress is updated at the end of a stage
- show 'checking' at the end of full sync
- all .ftl files for a language are concatenated into a single file
at build time
- all languages are included in the binary
- external ftl files placed in the ftl folder can override the
built-in definitions
- constants are automatically generated for each string key
- dropped the separate StringsGroup enum
