Show obj-to-emit white emitter is selected/open

Objects that are the target of an emitter (the thing that will be emitted) are automatically powered off. That's fine, and how it needs to work.

But this confuses new players all the time. They don't know how to see the thing it's going to emit, or edit it. They don't realise that it's actually still there in the scene using thermo; they often believe it's not there at all until it's emitted.

So they resort to things like removing the link (triangle on the "object to emit" button). This automatically restores the object's power setting and now they can see it. They make changes. They re-target it and the object hides again.

But of course, this means any adjustments they already made in position, orientation, and size of the emission can be lost and they have to put it back into place every time.

All they needed to do was to turn off "preview invisibility" and they'd see the object, understand it is still there, and be able to edit it, etc.

The proposed change is: show the obj-to-emit while you're inside its scope, or the emitter in question is selected, or its tweak menu is open (or possibly only one or the other, or behind a preferences setting... whatever). And this is regardless of if it's hidden because of the "hide tool," or powered off. This would mean it is a lot easier to find the obj-to-emit in the first place, make it clear it's still in the scene, and allow us to easily edit the object.

This would make it faster for advanced users to find and edit these objects, as well as make them easier to find and understand for newcomers.