Deprecate Tabs module for D7 in favor of Elements

Issue Summary

As maintainer of Tabs I plan to deprecate it for Drupal 7 in favour of tabs support in the Elements module, see #1034690: Add a horizontal tabs element.

Rationale:

• There's nothing special about tabs--they should be handled in a manner parallel to that of other UI elements like accordions etc.
• Elements should follow Drupal core where possible. In the case of tabs, there are vertical tabs in Drupal 7 core so a Drupal 7 version of horizontal tabs should closely emulate what's in core, for a parallel developer experience.
• Providing a tabs element in Elements could enable, e.g., integration modules for panels, views, etc. that are not specific to tabs but can work across a range of elements.
• The draft vertical tabs implementation for Elements is bare bones and simple, meaning it should be easy to maintain.
• I wrote tabs module for Drupal 4.6 IIRC and no longer provide more than minimal maintenance. No one else has stepped forward to take over. Much better a simple, stable UI element than a more complex and buggy one.
• The extras that tabs module adds - e.g., navigation buttons between tabs - would be better generalized to apply across element types.

Implications would include:

• Tabs no longer based on jQuery UI. See #1028424: Why a custom implementation rather than jQuery UI tabs?.
• Work needed to get the horizontal tabs element into Elements.
• D7 versions of modules building off of tabs would require Elements instead of tabs and need updating to the vertical tabs structure. Ideally, modules like views_tabs and panels_tabs_style would be renamed and rewritten to support multiple element types.
• Some functionality would disappear, e.g., navigation buttons on tabs.

Please comment in this issue if you have thoughts. Thanks.