In creation of edit form check for 'edit' part of POST data

This prevents creation edit form in case POST data were filled from different sources (like filter).

Create a grid. Use filter (without this commit this triggers creation of edit form thus creating hidden input with edit[$primaryKey]) and now click edit (this downloads only requested row from the server including https://github.com/nextras/datagrid/blob/master/Nextras/Datagrid/Datagrid.latte#L103). You won't be able to save the edit because edit[$primaryKey] value will be overwritten by the edit[$primaryKey] created before.