Revert 6c6c0cbd4d95957c107e0e7e37b6b4cf70b04de5
MusicMetadata: copy m_actualFilename in operator=

This fixes the regression that was the root cause of #12088 and also a similar
problem in the import screen when trying to play the tracks there as well.

Refs #12088.
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.
AlbumArtImages: use the filename as is for non myth urls in findImages()
Merge remote-tracking branch 'upstream/master'