Update icon for editing alt text to agree with current CKEditor

Problem/Motivation

The icon for overwriting alternative text in the Drupal CKEditor WYSIWYG is an eye with a slash through it. Not only is this a disconcerting way to represent the inability to see, it is not particularly meaningful, which potentially reduces the chances of somebody actually using it to add or improve alternative text.

I personally did not recognize that it represented the ability to override alternative text and had to have it pointed out to me. I'll note that I am icon-challenged, so icons need to be very clear in order for me to be able to use them fluently.

CKEditor is currently using an icon of the letters ALT over a picture. This is much clearer and doesn't have any disconcerting connotations.

Please update to the current icon.

Steps to reproduce

1. Go to simplytest.me
2. In the Evaluate Drupal projects field, type Drupal.
3. In the auto suggest drop down, choose Drupal core.
4. Choose 11.2.x-dev
5. Click Launch sandbox
6. Log into the instance
7. Click Extend
8. Type Media in the search field
9. Check Media and Media Library
10. Click Configure
11. Click Text formats
12. Click Configure on the row for Basic HTML
13. Drag the media icon to the toolbar
14. Under Enabled filters, check Embed media
15. Under Filter settings, Embed media, check Media library
16. Click Content
17. Click Add content
18. Click Basic page
19. In the WYSIWYG tool bar, click the image icon
20. Go through the procedure to insert an image
21. Click the image in the WYSIWYG

Desired result: The icon for editing the alternative text uses the current CKEditor icon of the letters ALT over a picture

Current result: The icon for editing the alternative text uses an eye with a slash through it

22. In the WYSIWYG tool bar, click the media icon
23. Go through the procedure to insert an image
24. Click the image in the WYSIWYG

Desired result: The icon for overriding the alternative text uses the current CKEditor icon of the letters ALT over a picture

Current result: The icon for overriding the alternative text uses an eye with a slash through it

By way of comparison:

1. Go to the Feature-rich editor demo on the CKEditor website
2. Click the image icon
3. Go through the procedure to upload an image
4. Click the image in the WYSIWYG area

Desired and current results: The icon for overriding the alternative text uses the letters ALT over a picture

Proposed resolution

Replace the slashed eye icon with the current CKEditor icon of the letters ALT over a picture.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet