More consistent icons in place of some buttons for the Customizer and theme install preview.

* Use a single close icon instead of a context-changing cancel/close button in the Customizer. In user testing, this small distinction in language was overlooked, and we use an AYS now instead. This also makes it consistent with the back arrow in Customizer panels.
* Use the same close icon as well as the previous/next arrows from the theme details browser in the theme install preview.

props celloexpressions. fixes #28655.

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git-svn-id: http://core.svn.wordpress.org/trunk@28819 1a063a9b-81f0-0310-95a4-ce76da25c4cd