Themes: update allowed theme tags to add flexible-header

[lancewillett]

Update the "wporg_theme_feature_list" list of available theme tags to include 'flexible-header' — a feature term to denote themes that support the new WP 3.4 flexible custom header header feature.

Patch also adds new term 'flexible-header' term to Twenty Ten and Twenty Eleven stylesheets.

(Previously: #17359)

[lancewillett]

Cross-posted to ​http://make.wordpress.org/themes/2012/07/18/id-love-your-thoughts-on-adding-two-new/ to get input from Theme Review Team on adding these two new tags.

We've been using them both for a while now on WP.com, and I think it makes sense for everyone to start using them, and for plugins like Theme Check to recognize as valid—and the Extend uploader script, of course.

[ramiy]

Maybe related: #21256

[lancewillett]

After discussion on Make Themes, I think a better approach is to rename "Widths" terms to "Layout" and change the three allowed values to "Fixed, Fluid, Resonsive".

[lancewillett]

Split out new term changes for layout/width to #21442 so we can get flexible-header in now.

[lancewillett]

In [21533]:

Twenty Twelve: add flexible-header tag back into style.css, in anticipation of it being a core-allowed tag, see #21065.

Remove responsive-width since it's not decided yet, see #21065.

[lancewillett]

Ready for commit, please.

[nacin]

In [21604]:

Add new 'flexible-header' tag for themes. Add it to Twenty Ten and Twenty Eleven. [21533] added it for Twenty Twelve.

props lancewillett. fixes #21065.