#2344 gallery enhancement

[lfcnassif]

Closes #2344.

IndexReader/IndexSearcher.doc(id) calls are heavy, moving them from AWT thread to thread pool makes gallery cell rendering parallel and non AWT blocking on spinning disks.

@wladimirleite if you could confirm it improved rendering on spinning disk, I would appreciate very much. I also copied your improvements on other classes.

[wladimirleite]

Great! I didn't try moving that call.
I will test tomorrow.

[wladimirleite]

I repeated the tests more carefully today, and the new version seems better, both using slow and fast disks. For the slow disk, the responsiveness (when scrolling the gallery with a lot of thumbnails) was improved. Thanks @lfcnassif!

I will push some minor code changes I made before, separated in small commits, so you can easily check them. Most are related to cache access. As the cache is very small (1000 items), there is no practical impact.

[lfcnassif]

Thanks @wladimirleite! A last detail I observed yesterday, just after cherry picking your commits, the "..." loading label stopped to be shown in the center of the cells. Could you take a look? I'm in a training this whole week and I shouldn't have time soon...

[wladimirleite]

A last detail I observed yesterday, just after cherry picking your commits, the "..." loading label stopped to be shown in the center of the cells. Could you take a look?

Sorry, there was a missing super.updateUI(). It should be working now.

[lfcnassif]

Thank you @wladimirleite!