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.
Built from https://develop.svn.wordpress.org/trunk@29903


git-svn-id: http://core.svn.wordpress.org/trunk@29658 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Theme install: Remove unused strings.

fixes #29488.
Built from https://develop.svn.wordpress.org/trunk@29904


git-svn-id: http://core.svn.wordpress.org/trunk@29659 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Customizer: Only POST dirty settings to preview to improve performance.

props westonruter.
fixes #29983.
Built from https://develop.svn.wordpress.org/trunk@29905


git-svn-id: http://core.svn.wordpress.org/trunk@29660 1a063a9b-81f0-0310-95a4-ce76da25c4cd