Fixes jameelmoses/acf-flexible-content-extended#11

[romanklabal]

This pull request fixes issue #11

To quickly test it, you can do the following:

1. Create a new Field Group, call it First, add a Flexible Content field to it with a layout called first-one
2. Verify that your plugin correctly shows the preview for first-one.jpg
3. Duplicate the First Field Group using the inline action, rename it to Second, rename the layout from first-one to second-one
4. If you test it now, you will see that one of the previews (either first-one.jpg or second-one.jpg) will be missing because the layouts have the same internal key and it has been skipped.

My PR solves this by combining the layout key with the flexible content key.

[jameelmoses]

Approved. Thanks for contributing! I'll draft up a new release as soon as I can; likely toward the end of this week.