Trac: attachment links need text

[sabernhardt]

Accessibility issues:

1. The Raw Attachment download links do not have any text, and they could include the file name to make the text unique.
2. The image previews do not have alternative text.

This can be improved, but it's a start.

• "Raw attachment" and the file name are added as invisible text for download links, at least within the comments area. (I kept the "Download" title attribute, though it does not help everyone.)
• The filename is inside its own container for use in the preview links.
• The preview images could use the filename as alternative text in all cases, but I favored the user-added description—when available—in this patch.

https://meta.trac.wordpress.org/ticket/6319

• "Download" and the file name are added as visually hidden text for raw attachment download links, at least within the comments area. (The "Download" title attribute is removed.)
• The filename is inside its own container for use in the preview links. (The class could be applied to the first link instead, if preferable.)
• The preview images could use the filename as alternative text in all cases, but I favored the user-added description—when available—in this patch.

https://meta.trac.wordpress.org/ticket/6319