Feature: unselect back into nodes that were selected out of

Behavior change: keyboard select (Shift-Left/Shift-Right) will unselect back into nodes that were selected out of, hence, call `unselectInto` on nodes that were `selectOutOf`-ed, which are detected by checking if the anticursor is inside the node.

Depends on `refactor.selection` #142, which isn't yet ready to merge, and neither is this.