Screenshot image alt attributes

[joedolson]

The image slideshow provides figcaption descriptions, but no alt attributes. I agree that under the current system the value provided is better presented as a caption than an alt attribute, but the alt attribute should still be present with an empty value to prevent the src value from potentially being read by screen readers.

The alt attribute in the thumbnail controls should be populated with the caption text, as well.

[obenland]

In 4469:

Plugin Directory: Add alt attributes to screenshots.

Fixes #2295.

[obenland]

In 4471:

Plugin Directory: Add alt attributes in PHP output.

See #2295.