Add internationalization support

* The framework specific messages (like "Zoom In/Out" or "Add point") are internationalized and translations were added for English, French and German.
* This PR also demonstrates how to translate a user form, the sample form is translated to English and French.
* The language is detected automatically from the browser settings. But the language can also be changed manually (with a link like `/locale?language=en`). The manually selected language is stored in a cookie.
* The makefile now also compiles the messages when running the `install` target.