When it is enabled, the project can scroll up to one-half of a screenful
beyond time zero or the maximum track time. I was careful to disable selection
of negative times.
This is motivated by the smooth scrolling scrub. It behaves more sensibly at
the extremes. It can still keep the play indicator centered.
Also removed an unused member of ViewInfo.
This change is believed to be a direct refactoring that does not change functionality. It paves the way for more complex kinds of selection, such as selections involving frequency as well as time. It also reduces risk of left and right edges being swapped in future code using SelectedRegion, as the default is to swap on assignment if needed.
