[LMP] More STL compatibility in Playlist for #1474.
[LMP] Playlist::IsEmpty() for #1474.
[LMP] Rely on Enqueue in ReplaceQueue for #1474.
[LMP] StaticPlaylistManager::GetCustomPlaylistPath() for #1474.
[LMP] Return static playlist path as a source for #1474.
[LMP] Respect playlist metadata on enqueuing.

Particularly, set current source according to the saved data.

Finally closes #1474.