libview: drop EV_JOB_THUMBNAIL_PIXBUF option, and all related code

Which includes:

• Getting rid of the ev_job_thumbnail_set_has_frame, since a comment in the code clearly states that it's incompatible with THUMBNAIL_SURFACE.
• Removing the widget property of the thumbnail job, that is no longer needed.
• Removing EvJobThumbnailFormat, since there's now a single format, and in consequence of the setter too.

EV_JOB_THUMBNAIL_SURFACE was introduced in b840a17b, as a more performant way to display thumbnails. The following commit, d47597cc, migrated the uses at the time to the surface, stating:

"Now that the sidebar model uses cairo surfaces, getting a surface from the backend we avoid an unnecessary conversion from GdkPixbuf to cairo surface."

That was in 2014. 9 years later, it's probably time to get rid of the old renderer, that is not used anywhere internally, and is anyway less performant. This also slightly simplifies the client code, that needs to do less stuff.

Co-developed-by: Qiu Wenbo qiuwenbo@kylinos.com.cn

There is an additional commit from !373 that can now be applied here.

Helps !373