GUI editor for compose keys combinations

[pseudomonas]

Hi, I wonder if you kind people could give me some advice. I make heavy use of the system for entering non-ASCII characters whereby ComposeKey, char1, char2 gives another character, so (for instance ComposeKey, "!", "?" gives "‽").

I would like to know if there are any GUIs for editing these combinations, so I can easily add a few. I do in theory know how to do it by editing the config file, but this means lots of looking up hex codes and risking breaking things (it's quite easy to create combinations that make other characters unavailable).

Searching only gives me lots of advice on how to remap the ComposeKey (I'm happy with it remaining at its default of Shift-AltGr). Ubuntu packages seem to lack anything with the obvious keywords :-/