Datagrid Aggregates for Columns without field

[oaldrian]

Is your feature request related to a problem? Please describe.
If I have columns that do not directly link to a field as sources for the data, but instead use Display and EditTemplates (#548), there is currently no way to attach an aggregate to this column, as the column identifier for the aggregate is the field name.

Describe the solution you'd like
Introduce a ColumnID parameter to the Column and the Aggregate that is set to Field, if it is not overridden. Use this ColumnID paramter to match the aggregate to the column.

Additional context

introducing an extra standard row to the grid, with the aggregates inside, works only if there is no sorting / filtering / grouping, so it is not really a viable workaround.

A current workaround is to create many (many) placeholder properties in the class to bind the dynamic columns to.