selection tool

first implementation: like eraser
- [x] drag across strokes to create a selection
- [x] style the selection: green
- if these operations are run, they operate only on the selection:
  - [ ] cut
  - [x] copy
  - [x] paste
- [x] dragging with shift will add to the current selection
- [x] dragging with alt will remove from the current selection
- [x] drawing, or pressing `D` will clear the selection
