Signal Callback Template

[cgrinker]

Perhaps part of #50 . Build a struct class for holding a pointer to an object, and the call back id to remove. Then build a bunch of helper methods around it, and replace it everywhere.

The point of this would be to reduce pain points (separate callback id variable, remembering to remove it on destruction, usage in collections, etc.). It would also be nice if it interacted in the declaration of signals (e.g. SObjectFungible could have an already ready to go one, or that the signature could be thingy<SObjectFungible::CB_onUpdate>.