Fix find bar display

Show and hide the find bar by adjusting the value of its height constraint, then tie the top of the web view to the bottom of the find bar.

Fix the bottom spacing of the elements in the find bar to create the effect of the bar sliding up. Performing an actual slide animation doesn't really work here since the focus ring draws on top of everything else.

Also use a shorter duration to get a bit closer to the animation used in Safari's find bar.