Widget Customizer: Improve behavior of bigger widgets.

* Support widgets which are higher than browsers viewport.
* Use widgets width as max-width.
* Don't animate the width of wide widgets to make them visible, instead fade them in/out.
* Fix Chrome flickerings while updating wide widgets.

props adamsilverstein, westonruter, ocean90.
see #27348.

git-svn-id: https://develop.svn.wordpress.org/trunk@27798 602fd350-edb4-49c9-b593-d223f7449a82