Twenty Themes: Add support for immediate Header text removal

[obenland]

There is a class that one can use in the admin header image callback, to support immediate removal of the header text, when the corresponding checkbox is checked, rather than waiting for saving the page.

Let's bring that to our default themes.

[lancewillett]

Nice one.

Note: this is only for Appearance > Header plugin page, right? Not Theme Customizer.

[lancewillett]

Related note: both Twelve and Thirteen already have JS handling for hiding the title/decription in Theme Customizer, with header_textcolor value via postMessage. Eleven does not have that capability.

[obenland]

Yes, this is in the admin header page.

[lancewillett]

Added #24128 for Twenty Eleven postMessage and header_textcolor enhancement.

[nacin]

In 24050:

Add .displaying-header-text to the default themes, as the Custom Header page toggles this class. props obenland, fixes #24113.