... and anticipate a problems fisheye will introduce with time ruler display
and horizontal scrolling past zero.
Will be used in waveform and spectrogram drawing, and in hit-test for sample
editing. Those things will no longer make the assumption of uniform zoom
level across the width of the screen, though that does remain true without the
rest of the fisheye project.
... 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().
