Skip-to-preview links for keyboard users in the Customizer

[celloexpressions]

See ​https://make.wordpress.org/accessibility/2015/01/31/theme-switcher-accessibility-test-result/. Let's add these links at the top and bottom of #customize-controls for now. We could also consider adding one to the preview, but since it would need to go into the footer (wp_footer) I'm not sure how useful that would be when dealing with inaccessible themes. Should probably wait on that part for now.

Patch is a start, but not yet functional, this needs someone to pick it up.

[celloexpressions]

The JS doesn't work yet, but looks like we do need JS here.

[afercia]

Replying to celloexpressions:

We could also consider adding one to the preview, but since it would need to go into the footer (wp_footer) I'm not sure how useful that would be when dealing with inaccessible themes.

Was thinking to use Escape as a sort of "go back" control:

• you're in the preview: press Esc to go back to the left pane
• you're in the left theme switcher pane: press Esc to close it and go back to the customizer main pane
• you're in the customizer main pane: press Esc to close the customizer

When nothing else works and you're trapped: press Escape :)
Not sure it can be done inside an iframe though.

[celloexpressions]

@afercia do you think we should still look at finding a way to improve this, or should we close this ticket for now?

[afercia]

@celloexpressions think we should close it :)

[celloexpressions]

We can revisit this in the future if we can come up with a strategy here.