Tristate checkboxes

Added new tristate checkbox API (with CheckBoxState).
Added tristate checkbox cell view API
Implemented new APIs in GTK backend.
Removed GtkMac tests from Linux-Debug configuration.

Also in Xwt/Xwt/CellView.cs, I add to replace a `== DBNull.Value` check with a `== null || == DBNull.Value` check, otherwise I'd get an exception there.