Enhance `Thumbs` plugin (#835)
Enhance `Thumbs` plugin (#835)

Improves the efficiency of thumbnail generation by using the timer thread with a new data structure to focus priority on building the thumbs in the currently visible set.

Also fixes a bug where the proper thumbnail was not highlighted when a new image is opened in the viewer.


ejeschke/ginga,naojsoft/ginga,naojsoft/ginga,pllim/ginga,pllim/ginga,ejeschke/ginga,ejeschke/ginga,naojsoft/ginga,pllim/ginga