Add media questions

[jmontoyaa]

Media questions are items in exercises that are stored as questions internally but that act as a grouping feature: you only give a media in a textarea, and then you can attach other questions to it.
When other questions are attached to it, they appear on the same page as the "Media". If one of the attached questions has to be shown, then the media question must be shown too (before the attached question).

[ywarnier]

To be a bit more specific, this includes:

• allowing a new type of question called "Media"
• this type of question does not have a score, because it is not really a question
• other questions can be attached to this media through a "parent_id" field, which was added to c_quiz_question in 1.11.x but could be added to resource_node and/or resource_link in 2.0
• when attaching a question to a media, the media and the question have to belong to the same category
• the media is never selected by the random selector, and is never shown "alone". It doesn't count as a question in the total questions count either.
• whenever displaying one question attached to the media, the other questions selected (either randomly or not) should appear on the same page if they are attached to the same media (grouping together), even if the test is on "one question per page". If the test is on "all questions on one page", then the media should appear first and then all questions attached to it.
• on the test results screen, all questions attached to a media should appear next to the media (grouped)
• the media question doesn't count in the total score nor in the calculation of the max score.

[ywarnier]

I have added #6306 to serve as inspiration : this is very old code initially developed for 1.9.x but (at the time) approved for production, that I tried to adapt as much as possible to 1.11.x.

This is to use to review what changes were applied back then and adapt to master. It does not have to be copied "as is", as maybe some changes do not apply to master. For example, the "parent_id" field could be replaced by a parent structure in resource_node.

This code was originally also mixed with a different questions "pagination" system which was not transfered to Chamilo 1.9 either, so maybe some of the code that has to do with categories and pagination remains in this PR and can be ignored.