Accessibility: Make sure layout tables across the admin are correctly linearized.

Adds `role="presentation"` to the `<table>` elements used for layout purposes.

Ideally, HTML tables should be used for tabular data. When tables are used for layout purposes, it's important to remove any native semantics so that assistive technologies can correctly announce the table content in a linearized fashion.

Props greatislander, afercia.
Fixes #46899.

git-svn-id: https://develop.svn.wordpress.org/trunk@45403 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/custom-background.php

@@ -246,7 +246,7 @@ class Custom_Background {
 
 <h3><?php _e( 'Background Image' ); ?></h3>
 
-<table class="form-table">
+<table class="form-table" role="presentation">
 <tbody>
 <tr>
 <th scope="row"><?php _e( 'Preview' ); ?></th>
@@ -340,7 +340,7 @@ class Custom_Background {
 
 <h3><?php _e( 'Display Options' ); ?></h3>
 <form method="post">
-<table class="form-table">
+<table class="form-table" role="presentation">
 <tbody>
 		<?php if ( get_background_image() ) : ?>
 <input name="background-preset" type="hidden" value="custom">