Use AJAX instead of dynamic PHP calls in iframes

[HugoFara]

LWT was built long ago when it was common practice to use only PHP, but time has changed.

The iframes system do not offer a lot of flexibility and integrates poorly in the modern web environment. They should be replaced by AJAX queries.

The new REST API (#145) is a good base to start.

[HugoFara]

Now, we expended the API to to operations on the front-end. The iframes are only used for dictionaries.