GridColumn now has Source dependency property that will be used as the GridCell.Content.  If Source is not specified, will fall back to the GridCell item.
No longer setting Content property in GridTemplateColumn as that work is done in GridColumn.
Created editable 2-way GridTextColumn.
Corrected dependency property registration.
Corrected GridTextColumn on demo.
Fixed sanity check.
Removed ActiveTemplate.  Moved EditStates visual states  to GridCell.  Fixed editing of items.
Added change notification to items so that 2 way bindings work in test.