Create WP_Customize_Control to separate the process of rendering a control...

Create WP_Customize_Control to separate the process of rendering a control from fetching, previewing, and saving its values. see #19910.

Many-to-many mapping between settings and controls.
* Settings and controls have been separated in both the PHP (WP_Customize_Setting, WP_Customize_Control) and the JS (wp.customize.Setting, wp.customize.Control).
* While most settings are tied to a single control, some require multiple controls. The 'header_textcolor' control is a good example: to hide the header text, header_textcolor is set to 'blank'.

Add 'Display Header Text' control.

A handful of miscellaneous bugfixes along the way.

Notes:
* Controls should be separated out a bit more; juggling type-specific arguments in the switch statement is rather inelegant.
* Page dropdowns are currently inactive and need to be re-linked.

git-svn-id: https://core.svn.wordpress.org/trunk@20295 1a063a9b-81f0-0310-95a4-ce76da25c4cd