Filip Roséen authored Jul 23, 2016 and François Cartegnie committed Jul 24, 2016

Given that one should always be able to sort the playlist, aborting sort
on "indexes.empty()" is inaccurate since we should not mandate that:

 - a user either right-clicks on an entity in order to sort, or;
 - has at least one index selected.

 This commit fixes the above described issue, as well as the arguments
 passed to PLModel::sort. More specifically, that the first argument is
 the first index (if any), and the second being the root index.

Signed-off-by: Francois Cartegnie <fcvlcdev@free.fr>