When moving symbol class between headers, allow placement into last place

[wayfaringrob]

If you try to move a symbol class up into the header group above, it is currently impossible to place it last, which may be desired. You have to put it above the last symbol class and then move it again to where you want it - another unnecessary extra click in an application seemingly built around unnecessary extra clicks. Surely, it's a feature, not a bug, right? /s

(Below: the '6' cannot be dropped below the '5').