Cast datagrid-editable-value to string

[Rixafy]

There was a problem, when someone uses setEditableValueCallback on a row, and returns some string, but this string is for instance 123, javascript casts that attribute to integer, and when you click away, callback is called because integer != string - datagrid thinks the value has changed.

[radimvaculik]

Good for me! 👍 @f3l1x / @paveljanda Could you merge it, please?

[paveljanda]

Thanks