Make changing a page more visible with an effect on the thumbnails

[daniellienert]

When changing the page using the bottom paginator, the thumbnails are kept until the backend fetch is completed.
When the thumbnails are being generated and the backend fetch takes some time, the user gets no visible feedback of the page change.

It would help to have some spinner on the thumbnails or blurred images to make the loading visible.

[Sebobo]

Started playing with that already but then stopped again due to lower priority. But yes we should have that.