Fix #1483 by tweaking hide/unhide techinque for menu.

Now, instead of fadeOut/fadeIn, we animate opacity to and from 0 and when the menu is invisible, we detach it from layout (position:absolute) and move it out of the way of the expanding search box.
Merge pull request #1558 from NuGet/anurse/1483-searchboxissues

Fix #1483 by tweaking hide/unhide techinque for menu.