reset z-index for cke toolbar in a slideout

[i-just]

Description

Fixes an issue where if you had two CKE fields, one under the other (first one with very little or no content), and you expanded a dropdown in the first field’s toolbar, that dropdown would go under the toolbar of the second field, but only if the element containing the fields was viewed in a slideout.

Before:

After:

Related issues

#376