dnd needs a clean action

[albfan]

Once you set left or right panel with dnd, it keeps files or directories forever.

That makes confusing to create new comparisons with dnd

[albfan]

a possible solution on #7 cfee554

not much intuitive, but if we want to keep one of the files and change other files for comparison, clean all the time make it more difficult. in that case a clean action is still needed.

Maybe Ctrl+Click may modify and trigger clean