Refactoring

Hi Nicolas,

We not only really need this plugin, but also this to enable CLI scripts using the merging utility. To do so I have refactorized the plugin and updated internationalization string to that goal.

In addition, I have updated the result to be more Moodle-friendly (both in web and code).

In the last, but not least, I have added new special cases for the database tables related to Moodle messaging, not treated before. 

There is also a new feature: in the previous version, we only processed tables with only a user-related field. Since there is a message-related table with two fields of that kind (userid and contactid), I have also expanded the application of the merging utility to every user-related field of the matching tables.

This is a very big change, I know. Hoping you will consider positively my/our contribution. You should think we have the resulting version of this PR in our exploitation Moodle instance.

Next contributions will be related, if you would like to accept them, to build a CLI script to make them automatically.

Since this is a major change, I bumped the version to 1.3 :-D

Thank you in advance,

Jordi