Keyboard focus on the header and background settings screens

[johnbillion]

On the 'Appearance -> Header' and 'Appearance -> Background' screens, there are two oddities when using the keyboard to tab around.

• The site title in the header preview is focusable, but it shouldn't be
• The 'Choose Image' button is not focusable so it's not possible to choose an existing image using keyboard navigation

[florianziegler]

Add href and tabindex attributes

• Add href attribute to "choose image" links to make them focusable.
• Add tabindex attribute and set it to -1 for preview link so it is not focusable.

[obenland]

Patch tested and works.
I also ran it by davidakennedy for the way how it is implemented and he checked off on it.

FWIW, if there is only one custom header uploaded, it is not possible to keyboard-select that custom header. This might just be a defect with single radio buttons though, not sure.

[florianziegler]

Keyboard selection of (only) one custom header works for me. (Tested with the latest Chrome, Firefox and Safari.)

Am I missing something?

[ocean90]

In 30327:

Improve keyboard accessibility on Custom Header and Custom Background screen.

props florianziegler.
fixes #29289.

[ocean90]

In 30351:

Change Twenty Eleven's header preview callback too, missed in [30327].

see #29289.