Accessibility: improve the "URL" and "Alt text" fields in the media modals.

Many users found the attachment URL field confusing: it says "URL" so it may appear like a field meant to paste a URL into.
Also, the Alt text field is the most important one in terms of content, while the Title field needs to be de-emphasized.

- changes the URL field label to "Copy link"
- moves the alt text field to the top as first field 
- avoids to set initial focus on the alt text field 
- adds an explanatory text with a link pointing to the W3C "alt decision tree" tutorial 
- adds `aria-describedby` to target the explanatory text
- adjusts the CSS accordingly
- updates the QUnit index.html file

Props melchoyce, audrasjb, afercia.
Fixes #41612.


git-svn-id: https://develop.svn.wordpress.org/trunk@44900 602fd350-edb4-49c9-b593-d223f7449a82

src/js/media/views/attachment/details.js

@@ -42,23 +42,10 @@ Details = Attachment.extend(/** @lends wp.media.view.Attachment.Details.prototyp
 			rerenderOnModelChange: false
 		});
 
-		this.on( 'ready', this.initialFocus );
 		// Call 'initialize' directly on the parent class.
 		Attachment.prototype.initialize.apply( this, arguments );
 	},
 
-	initialFocus: function() {
-		if ( ! wp.media.isTouchDevice ) {
-			/*
-			Previously focused the first ':input' (the readonly URL text field).
-			Since the first ':input' is now a button (delete/trash): when pressing
-			spacebar on an attachment, Firefox fires deleteAttachment/trashAttachment
-			as soon as focus is moved. Explicitly target the first text field for now.
-			@todo change initial focus logic, also for accessibility.
-			*/
-			this.$( 'input[type="text"]' ).eq( 0 ).focus();
-		}
-	},
 	/**
 	 * @param {Object} event
 	 */