Update CKEditor button images

[herbdool]

Description of the bug
The CKeditor buttons in the admin settings look different from the front end. I first noticed it with the Table button and the Source button.

The admin page:

Add a node form:

I viewed this in 1.14.x and 1.13.x.

[ghost]

Hmm, interesting! Confirmed in latest 1.18.x...

[indigoxela]

It seems like the "admin buttons" in core/modules/ckeditor/images/buttons/ should be updated to the recent versions from core/misc/ckeditor/plugins. 😉

[ghost]

I started doing this PR, only to get to the end of the images and realise that the source I was using didn't have all the ones I needed... So adding my PR here so as not to lose all my work, but this still needs work.

Going to change this from a bug to a task, since it's just a visual difference.

[indigoxela]

only to get to the end of the images and realise that the source I was using didn't have all the ones I needed

I'm not sure, but it might be that CKEditor is using sprite images.

[ghost]

Yes, that's what I used for the PR changes, but there aren't as many images there as are in https://github.com/backdrop/backdrop/tree/1.x/core/modules/ckeditor/images/buttons...

[indigoxela]

but there aren't as many images there as are in core/modules/ckeditor/images/buttons...

I see. Maybe they're not in use anymore? But it's probably easiest if you replace the ones you have available in the sprite and keep the other (older) ones.