Add support to "Save content state" in mod_h5pactivity

Implement the required classes to use xAPI state get/post inside mod_h5p in order to save the content state.

This implementation requires:

• Override the validate_state inside \mod_h5p\xapi_handler.php to accept states inside the activity
• Add support for state saving and restoration from the H5P player.
• Behat tests for saving and restoring states inside an H5P activity