`AllowDeselectionDuringDrag` is remove.
Instead, selected hit objects are not automatically deselected
when clock is seeked to a later time (the hit object is dead).
Update drag box selection even if mouse is not moved
(in case clock is running or scroll wheel is used).
Until now, the implementation of the overrides in `SelectionBlueprint`
have been confusing to the point where I would just implement by
trial-and-error (or copying from an existing implementation). This was
due to a combination of using "object" space coordinates
(ie. the thing the `Blueprint` is operating on) and screen-space coordinates.
This change switches all event related coordinates to screen-space,
which is how we already handle rotation/scale operations. With the
introduction of other editor types where the related objects are
drawables, this also makes a lot more sense.