distinguish sequence associated from non-sequence associated annotation in column filter dialog

The column filter and annotation colour dialog generates combobox elements for annotation rows based on their label. If many have the same name, as is usual for sequence annotation, then the labels have _i appended to distinguish them.

Since no visual indication is given as to which annotation row has been selected, the user can't distinguish annotation from a sequence from annotation on the alignment. Having more information in the label would be a start, but we could also do other things.