MythMusic: fix the playlist editor adding duplicate tracks to the playlist

Selecting some nodes more than once in the playlist editor tree would add
duplicate tracks to the active playlist and the tree because we wasn't checking
if we had already added the tracks for the selected node.

(cherry picked from commit c907a46d2604382b3bad6e44d4574b91e7692bf7)
Merge branch 'fixes/0.27' into stable/0.27