Saving a session registers all commands reducing reliability

[Tom Goddard]

When saving a session it calls core.commands.list_aliases() and that appears to register every single ChimeraX command. Usually commands are only registered when used. Registering all the commands imports a huge number of files and can sometimes lead to crashes as seen in ticket #5332 where a session save causes the obscure webcam command to be registered which imports QtMultimedia for handling video cameras (the most massive Qt library), possibly wiping out on dynamic linking problems.

It does not seem like session saving should register every command. The purpose is to capture user defined aliases in the session file. No commands should be registered to achieve that.