Theme Customizer: Numerous API refinements and bugfixes. Add a theme_supports...

Theme Customizer: Numerous API refinements and bugfixes. Add a theme_supports check for header_textcolor. see #19910.

* prepare_controls() now removes any settings and sections that return false for check_capabilities().
* Added maybe_render() methods to both settings and sections that call the protected render() methods.
* Stop firing front-end preview functionality when rendering the controls.
* Merged the WP_Customize_Setting->_render_type() method into WP_Customize_Setting->render().
* Removed the 'customize_render_control-' hook; use 'customize_render_setting' instead.
* Added a  property to sections and settings so they no longer rely on the  global. Hooray for dependency injection.
* Shifted calls to WP_Customize_Setting->enqueue() to the 'customize_controls_enqueue_scripts' action.
* Added a theme_supports check for the header_textcolor setting.

git-svn-id: https://core.svn.wordpress.org/trunk@20248 1a063a9b-81f0-0310-95a4-ce76da25c4cd