Add and start using an object set

Where equivalence is based on identity.
Add game execution toggling
Force the GUI to respond properly to a SIGINT
Add sequence limitation test
Rename list_before

It doesn't need an "after".
Make frontier remember seen objects

Previously loops could occur when an object was popped and then
re-added.
Reduce dependency on pygame in reloader
Make list_loop example work

Also added a time delay so that the update can be seen to occur.
Totally make lists work
Make it do all the things I'm saying it can do in the report
Continue making it do the things
Remove some prints