Accessibility: Improve accessibility of all the media views form controls.

- changes the media views form controls to have explicitly associated labels with for/id attributes
- adds a few missing labels / aria-labels
- improves a few existing labels / aria-labels
- improves semantics in a few places, by adding visually hidden headings, fieldset + legend elements, aria-describedby attributes
- improves the image custom size input fields and their labelling
- adds `role="status"` to the "saved" indicator so that status messages are announced to assistive technologies
- swaps the columns source order in the image details template, to make visual and DOM order match
- swaps the "Replace" and "Back" buttons source order in the Replace Image view, to make visual and DOM order match
- gallery settings: move checkbox label to the right: checkboxes are supposed to have labels on the right
- merge similar strings, unified to "Drop files to upload" (removed "Drop files here", and "Drop files anywhere to upload")
- makes the "upload-ui" consistent across the media views
- hides the IE 11 "X" `::-ms-clear` button in the Insert from URL field, as it conflicts with the uploading spinner
- adds comments to all the media templates to clarify their usage
- slightly increases vertical spacing between form fields in the media sidebar
- removes some CSS selectors introduced as backwards compatibility for WordPress pre-4.4
- removes some CSS still targeting Internet Explorer 7 and 8

Fixes #47141.
Fixes #47122.


git-svn-id: https://develop.svn.wordpress.org/trunk@45499 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/media.php

@@ -2072,7 +2072,7 @@ wpUploaderInit = <?php echo wp_json_encode( $plupload_init ); ?>;
 	?>
 <div id="drag-drop-area">
 	<div class="drag-drop-inside">
-	<p class="drag-drop-info"><?php _e( 'Drop files here' ); ?></p>
+	<p class="drag-drop-info"><?php _e( 'Drop files to upload' ); ?></p>
 	<p><?php _ex( 'or', 'Uploader: Drop files here - or - Select Files' ); ?></p>
 	<p class="drag-drop-buttons"><input id="plupload-browse-button" type="button" value="<?php esc_attr_e( 'Select Files' ); ?>" class="button" /></p>
 	</div>