Tooltips for image buttons

[Akuli]

Would be nice if the pause button displayed "pause" when hovered, and similarly for other buttons. A tooltip (or a different image) would be especially useful for the "X" button, because it isn't clear whether it deletes the output when clicked, but it would be clear if it had a tooltip saying "Hide output area" or similar.

Porcupine used to have code for creating tooltips, but it was a lot of code to maintain for only one tooltip in the whole Porcupine, so it was deleted in #610. We can revert that PR to get it back (and maybe edit the docstring to be slightly less opinionated towards idlelib).