Gsoc central columns list

Commits contains:
On db_structure page: With Selected "Add unique columns to central list"
                                With Selected "Remove columns from central list"
On tbl_structure page: With Selected "Add columns to central columns" 
                               With Selected "Remove from central columns"
                               In individual columns row, add/remove to/from central columns depending upon  columns is present in central list or not

added doc for configuration storage of central_columns
regenerated sprite after adding two new icons for central list of columns