Gallery image: fix the elements order

[afercia]

Noticed while testing #14822

In the gallery images, the order of the elements should be improved. Currently, the order is:

• "Remove Image" button (see block-library-gallery-item__inline-menu)
• the image (which is focusable)
• the caption field

With this order, there are at least 2 issues:

• when using a keyboard, the "Remove Image" button gets skipped because it appears after the image is focused
• even if it wasn't skipped, the "Remove Image" button doesn't give any context on which image it will remove because it's before any relevant information related to the image

I'd propose to:

• move the "Remove Image" button after the image: shouldn't be a big problem as it's absolutely positioned
• make sure the button appears both when tabbing forward and when tabbing backwards

[oandregal]

Closing this in favor of pre-existing #14814 I went ahead and added your feedback there.

[oandregal]

Also: there is a PR at #14821 to solve that issue. Looking for some feedback there :)

[afercia]

Thanks @nosolosw ! Was searching through the "Accessibility" label and missed it. Added the label there.