Toggle button for USNOB layer

[Carifio24]

This PR is intended to take care of the last item from #2 by adding a button allowing the user to toggle between Tycho and USNOB. @patudom feel free to change the text.

I also moved the tooltip location for these buttons to the top, because having it as start means that the tooltip for any button but the leftmost would overlap with another button.

[patudom]

I pushed a commit to also take you to T CrB if you're not already there - otherwise you wouldn't notice the change. There's something wonky happening, though. If you move T CrB off screen and click the new button, it will go to T CrB, toggle the layer to the opposite one briefly, then toggle it back to what it was before.

If T CrB is already on screen, it toggles to the opposite layer and stays there as it should, so I'm wondering if there is something about the timing that is confusing the app about what layer it's supposed to be switching to. @Carifio24, can you please take a look and see if you can set that up better than I did? Thanks!

What we really want to do is check how far away we are from T CrB, and not have any delay if T CrB is already on screen, but that seemed like a hassle, so I just picked a value that wouldn't feel awful to wait for if you're already there.

[Carifio24]

@patudom I pushed a commit to remove the weird toggling behavior by using the goto promise instead. (Note that we could also toggle the imageset before the move. Additionally, this button now moves instantly if T CrB is onscreen. Does this seem like what you had in mind?