WIP: Add drag & drop

This is still WIP. Checklist:

- [x] When dragging entries, create half-transparent copy and update its position on mousemove
- [ ] Only start dragging when mouse moved at least 5 pixels after the mousedown event
- [ ] While dragging, highlight the currently selected directory view
- [ ] On mouseup, perform file operations and re-render the tree-view
- [ ] Stop drag & drop on `atom:cancel`