Refactor apps' H5P code to make it more similar to H5P's PHP and Moodle structure

The first implementation of the H5P code was supposed to be temporary until we decided how to handle multiple versions of H5P in Moodle, so I chose the simplest approach and I put most of the code in a single provider.

Now that this code will be permanent it should be split into different classes, preferably using a structure similar to H5P's PHP and Moodle code to make it easier to compare them and detect changes.