UI: Those blocks are non-functional.
UI: Do not wait for image to fail in thumbs mode to show placeholder with extension.
UI: Restore progress feedback.

Why did I remove it in first place?.. There should be unit tests all
over the place. What you mean they are boring?!
UI: Adapt lazyloading on thumbs view. As suggested in #834.

Introduces huge relief on CPU for huge image sets.
Merge origin/master.