Replace addSystem() and removeSystem().

[player-03]

I think the new functions are much clearer, especially given that they're now in System where people are likely to look for them. But this is also a massive breaking change.

Before:

var system = new MySystem();
Echoes.addSystem(system);
//...
Echoes.removeSystem(system);

After:

var system = new MySystem();
system.activate();
//...
system.deactivate();

I could of course leave the old functions there, but honestly, they've always felt a bit out of place.

[player-03]

@NitroPlum What do you think of this?

Edit: sorry, pinged the wrong person.

[NitroPlum]

Yeah that makes sense. It's breaking but realistically it's easy and way more concise.