Improved filtering for DataGridCheckColumn

[mtbayley]

When the DataGrid column is a DataGridCheckColumn, only DataGridColumnFilterMethod.Equals makes sense

Additionally, I think it would be nice to select from true/false instead of typing "true" or "false" (many of our users won't know to do this).

I realize you could likely achieve this with FilterMenuTemplate, but I'm hoping this can be a default.