Add the possibility to filter orphans

In the table, the only way to find orphan guides & modifs is to sort the table in reverse order in the appropriate column to put the empty columns at the top (i.e. sort Guide name to have the orphan modifs with no guide in the column at the top).

Add the possibility to filter a column to only show empty cells.

On the Statistics page, the X orphan guide|modif text could then link to the table with those filters applied.