keyboard: By default, move selection when user sets keyboard target

[pokey]

Is a bit more intuitive than effectively having two cursors

For cases where you want a separate cursor that doesn't move your existing cursor (eg for "bring" or "chuck"), you first issue a command that detaches keyboard target. Then selection doesn't follow keyboard target anymore until the next action you issue

We'd probably get rid of the purple cursor in cases where it's following the selection

Keep in mind we'd have a hidden keyboard target then, because we can't just use selection because selection isn't a rich target