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.
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git-svn-id: http://core.svn.wordpress.org/trunk@27633 1a063a9b-81f0-0310-95a4-ce76da25c4cd