MDL-66402 core_h5p: adapted code for deploying H5P

The code added by Bas in the manager.php file has been moved to the
player file (we can decide later whether that's the correct location or
we should move to any other place like api.php/manager.php or similar).

There is still a lot of code to review (all the required for displaying
the H5P content). Besides, some TODOs have been added in order to take
into account some tasks/checks.