Use Cases and Requirements table, rev.4

Dear all,

Following the addition of two new use cases (UC-14:  User Control of Audio and UC-15: Video commentary) as well as one new requirement, I have updated the table mapping use cases and requirements. You will find it attached.

Other than the additions mentioned above, the only change to the table is that the row for "dynamic range compression", which had been TBA for all that while, is now filled. Matching it with our use cases gives it a priority of "Minority, but important" since it matches a number of HIGH priority use cases. 

Cheers,
Olivier