Customizer: Change instances of "Theme Customizer" to just "Customizer", as the Customizer isn't necessarily theme-specific.

Also capitalize "Customizer".
See also https://make.wordpress.org/core/2014/07/08/customizer-improvements-in-4-0/.

props studionashvegas, tareq1988.
fixes #29947.

git-svn-id: https://develop.svn.wordpress.org/trunk@29903 602fd350-edb4-49c9-b593-d223f7449a82