Fix #820: Override cell in CompoundColumns to find columns by original column id

This update adds an override to `cell` in `CompoundColumns` to handle the way column ids on child cells are prefixed with the parent's id.  Given a column configuration like this:
```javascript
{
    label: "compound",
    children:[
        { label: "Data", field: "data", id: "myData" }
    ]
}
```
A cell can be found using `grid.cell(rowId, "myData")` or `grid.cell(rowId, "compound-myData")`.

I wrote a unit test and fixed two problems I found while running the test.
* Compound columns configured with an id were not being prefixed with their parents' ids.  This would cause problems with the `ColumnSet` mixin.
* The `CompoundColumns` extension did not support the object column configuration.