A11y: New List Table headings in wp-admin Groups

[mercime]

WP 4.4-trunk, BP 2.5-trunk.

As outlined in ​make.wordpress.org/core, new headings have been added to improve accessibility for screens with List Tables.

Legend:
(o) changed in #6581
(+) add new heading

Headings:
(o) <h1> Groups <h1>
(+) <h2> Filter groups list </h2> (screen reader only)
(+) <h2> Groups list navigation </h2> (screen reader only)
(+) <h2> Groups list </h2> (screen reader only)

Screenshot is attached to show the new headings for the Groups screen in dashboard.

[mercime]

6742.patch adds accessible hidden headings and text for the BP_Groups_List_Table.

[mercime]

Updated patch fixes errors in WP 3.9.

[mercime]

In 10462:

A11y: Add headings for screen readers in the Groups list table panel.

Fixes #6742.