When using load() it gives items one at a time to addMedia, this should instead pass a list

When using the load() method of a Playlist for the MediaPlayer, it was found that performance is slow due to the readItems method that load uses passing each item to addMedia individually.

Instead this should pass the items as a list, please find attached a patch which changes this.

Let me know if you would like the code submitted in a different way such as a branch?

Note, however, that when testing with qtubuntu-media I'm still getting individual signals for each addition instead of one, but the performance has greatly improved (from ~0.2s for each item to add, to nearly instant for 100 additions) . I'm wondering if I'm using the wrong playlist() with the addMedia ? or if there needs to be a tweak within qtubuntu-media?