Mark fields as translatable

This is an experimental branch that displays label boxes "translatable" and "untranslatable" along custom fields.

The ```<span class="translatable">Translatable</span>``` is placed in the ```<label>``` directly after the label text, but I don't think that's a good place in HTML and the design isn't that fitting either.

But you can get the idea.

This work is based on https://github.com/widescape/engine/commit/0eaa0bef312b1bdfce5f4b2ed70cf5485383f036