... miscellaneous direct uses of ZoomInfo::zoom to test and set zoom level.
This includes all the remaining assignments to it.
But moving TrackInfo::PositionToTime and TrackInfo::TimeToPosition into
ZoomInfo and using them is needed to eliminate many more uses.
Also #if'd out the unused AudacityProject::OnZoomToggle().
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.
