Updates for xhtml style and gadgets

Gadgets:

* Support `changed` flag from `erp5.js`: 

![unsaved changes sreenshot](/uploads/cff30eae0a5d87df26bac13d43e1da3f/image.png)

* Comply with translation protocol used in ERP5JS. This does not translate, because it would not be easy ( ERP5JS translate client side with a json database of message, but in xhtml style the translation is performed server side with localizer) and not so important now. What matters is just that we can somehow reuse gadgets.

Style:

* While we are at it, fix a minor annoyance that the button in "long" dialogs where not easy to reach, at least on chrome

![before](/uploads/e03a8311571806a55fd72f6923cf855a/Screenshot_2019-05-31_at_10.38.05.png)

![after](/uploads/5753bc44fbc0f69b6adfeb14b3367236/Screenshot_2019-05-31_at_10.38.37.png)

/reviewed-on !880