medialibrary content can't be drag'n dropped to the playlist when detached

the issue is that drag'n dropping ML items uses internal DnD mechanism (vs system mechanism), as a result this can't get across windows.

I'm not sure what would be the best approach:

• using system solution only: we would probably lose the look'n feel. or

• using a mixed solution: something like using the system mechanism once we leave the window area: probably a bit tedious to keep track of the state:

• using internal solution when the playlist is docked and system when it's detached, this may work as long as the playlist is the only destination, and DND behavior need to be properly factorized to avoid having conditional code everywhere.