COMBINE statement uses current SEQUENCE_REF and GROUP_REF scope to group annotation rows

In previous Jalviews, the COMBINE statement operated to group every annotation row with the given identifiers (ie row label). This mean't that annotation rows with the same identifier but different sequence and/or group references were grouped together, making it impossible to save and restore protein disorder annotation rows (see JAL-1294).

Instead, COMBINE should honour any SEQUENCE_REF or GROUP_REF statements that precede it, and so only combine annotation rows with the same SEQUENCE_REF or GROUP_REF.