Code tooltips hang around too long

The code tool tips hang around too long, and can build up over time:

Pressing escape clears them, but it gets a bit irritating. Also, for overloaded functions, it interferes with the use of the up and down arrows for navigation (note in the video, once the `strpbrk` tip appears I switch to using the left and right arrows; that was to avoid this in the example).

I think a good solution would be to clear the tooltips when the cursor moves to a different expression. (Personally, I'd be OK with clearing them if the cursor moves to a different line, too, if that's the only option - even though "line" != "expression", I'd take those quirks over this one).