Separate selection operations into their own temporary history mechanism.
The user can now replay *all* earlier selection operations until the next
session undo/redo command, or the completion of a new operation.
Nothing relating to selection ops is stored, and selection operation history
is begun on first idle.

Selection operation history is fundamentally different from the history of
operations which act on a selection in terms of both their viewport and the
amount of information required to replay them.
WRT undo, the user of a selection op doesn't care about the viewport state
at the beginning of an op, but rather that at the end of the previous one.
Removed unrelated change in Selection::set_state () (clear_points()).