Make Alt+Drag copy selection

[nkrkv]

Rationale

Some software uses the Alt+Drag gesture to make a copy of the selected items along with moving them to the new place. XOD IDE can benefit from the same mechanics too, adding a new way to quickly replicate similar clusters.

User story

• [ ] I select a few nodes
• [ ] I hold down the Alt key (Option on Mac)
• [ ] I drag the selection
• [ ] The original selection stays in place
• [ ] A new copy of all the nodes selected with links between them moves casting the blue shadows
• [ ] I drop the selection
• [ ] The new copy is finally created at the point of the drop

Acceptance criteria

• [ ] The basic scenario works as described
• [ ] The nodes selected include links between them exactly like Copy/Paste do
• [ ] The comments are clonable too
• [ ] The Alt key behavior is added to the Shortcuts article (https://xod.io/docs/reference/shortcuts/)