Unify sensor API: no special access for built-in sensors

[narc0tiq]

If we move towards using the Factorio settings API, as per #76, we could also change things around such that the built-in sensors aren't just magically created already plugged into the core.

Instead it seems like the sensors should be set up differently -- each one would get a settings file (at minimum, defining whether it's never/always/popup visible) and a control file (with a register-at-init and whatever code necessary to display).

Meanwhile, the EvoGUI core would:

• allow for sensors to register/deregister
• create/destroy GUI panels for each sensor that wants to be visible
• track minimized/opened state
• tell sensors to update their GUIs (but not how, only where)

No more special magic settings window, no more super-magic on_click handling, and the sensors end up communicating probably through remote.calls (and being encapsulated, which should also allow creating one as example code).