Pressing Alt while dragging and dropping file between directories should copy instead of move

[carlosvsilva]

Is your feature request related to a problem?
When I have 2 panes open and I drag and drop a file from one pane to another Files always moves the file but sometimes I just want to copy

Describe the solution
When I'm dragging and dropping a file between Files panes if I press Alt when dropping Files should copy the file instead of moving.

Describe alternatives you've considered
I tried pressing Ctrl, Alt, Shift and nothing changed.

Additional comment
Files 1.5 and two panes side by side

[gave92]

Merging with #1396. Alt+Drag will probably make a shortcut though.. I believe windows explorer does the same.