support partial refresh in the customizer

[markkap]

partial refresh is a customizer api that enables it to send an ajax request to the server when the settings change in the customizer and the server can reply with the new HTML that should replace the "old" one in the preview.

We probably can't do changes like that for anything that adds different CSS or JS, but other settings like excerpts can be done this way.

Targeting for 4.7 even if we do only part of the possible functionality to integrate with the customizer shortcut feature that will show the user an icon that when its clicked, the appropriate secion in the customizer panel is opened.

[markkap]

relevant discussion with weston - http://wordpress.stackexchange.com/questions/247251/how-to-mix-partial-and-full-page-refresh-in-the-same-section-of-the-customizer

[markkap]

moving to 4,8 as I don't have the time to hack anything relevant and there doesn't seem to be a simple way

[markkap]

and moving again to 4.9 as again, do not have time :(