Remote sensor per-sensor settings

[narc0tiq]

As mentioned at #68 (comment), remote sensors should have the ability to register a per-sensor setting button, too. Keep it simple: they ask us for a settings button with a given name, we add the button and (optionally) pass the GUIElement back, then it's their problem to listen for on_gui_click on that button.

[originalfoo]

Button name should be sensor name - this way no need to pass GUI elements around or autogenerate button names. Will also make GUI event handling in the sensor mod much clearer as on_gui_click handler will be looking for sensor name.

[narc0tiq]

My reasoning for name mangling is to have namespacing -- there might already be a UI element by that name -- but equally I can have the remote sensor just tell me what the name should be and use the presence (or absence) of a requested button name to tell me whether to show one. I'll update the OP.