Display selected values at the top of the list when filtering in the column dropdown

When the list of possible values is very long, once some values are selected, the users will have difficulties skimming through the list to find or deselect the selected values.
Hence we should list the select values at the top of the filter in the column dropdown.

Some more thoughts need to go into the UI in particular if we have to handle hierarchical values (e.g. ontologies).

Start at the column dropdown entry point and inspect how filtering and selected values are rendered. Define the behavior for selected and hierarchical values, then verify that selected values remain easy to find and deselect in a long list.

Written by the indexing model from the issue text.