When we drag and drop a record in a folded column, the number of elements
in this column displayed in the column title is not updated.
This issue is present because when a record is droped in a column
its id is not added to the dataset of its new column.
To fix this, we had to add the id of the record in the dataset, and
override the method 'add_ids' in 'DataSetSearch' to have the size
of the dataset equal to the number of ids in it. We have replicated
the behaviour of the function 'remove_ids' in the new function 'add_ids'.
Pierre Masereel
8e65b57df7