* Add `Doc:get_selection_idx`
* Make multicursor paste add a cursor at the end of each paste
* Better manage paste of multicursor whole line copy
* Document `Doc:get_selection_idx`
* Keep track of last added selection in `Doc`
* Make use of `doc.last_selection` in `Doc` commands
* Make `Doc:get_selection` return the `Doc.last_selection` if possible