SongEditor Shift+Ctrl+Drag to Move Block

we have ctrl+drag that _copies_ a block from track a to track b
sh+ctrl+drag could then _move_ the block away from track a into track b
intuitive i think?

Start at the SongEditor drag handling for the existing Ctrl+drag copy behavior. Trace how a block is selected, copied between tracks, and released, then determine where Shift+Ctrl+drag should instead remove it from the source track. Done means the block moves to the destination track without remaining on the source, while ordinary Ctrl+drag still copies it.

Written by the indexing model from the issue text.