Added support for internationalization

[ejeschke]

This adds support and documentation for translating Ginga's strings into various languages through Python standards. As a beginning, a few example programs are internationalized for "en" (default) or "ja".

A chapter was added in the developers manual documentation explaining how to add a new translation.

This PR does not include translations for the reference viewer. The idea is to put in a small framework (this) in place, then later the reference viewer files and plugins can be converted on an as need basis (following the examples) in new PRs.