Make library column resizer more visible

The "Adjust column width" function is much easier to find and handle in the column header of the library.

(CMS-25982)

Enabled AMAsset Thumbnail Previews

Enabled the thumbnail previews for the document formulars of AMAsset and AMPictureAsset.

(CMS-25190)

Warning before publication workflow is fixed

When starting a publication workflow on contents with warning now shows correct warning messages.

(CMS-19538)