Work around mono crashing on Rows.Clear()

Mono will crash if you do a Rows.Clear() and the first row isnt visible.  To
work around this, when running under mono force scroll to [0,0] cell before
issuing the Clear().

https://bugzilla.xamarin.com/show_bug.cgi?id=24372

Renamed Settings.MonoFileIO to Settings.IsUnix
Created Settings.IsMono