Stop image thumbnail loading if thumbnails are not shown

[holgermappt]

Here is a patch that adds the function stopLoadThumbs() that stops geo image thumbnail loading if thumbnails are not needed anymore. The thumbnail load is continued the next time loadThumbs() is called. The JOSM revision needs to be inserted for function stopLoadThumbs() (@since tbd).

Please review the thread synchronization, my experience in that area is limited. I think a new thumbnail loader should wait if another loader is still running, but I don't know how that can be done.

One corner case is a sequence of setUseThumbs(true); setUseThumbs(false); setUseThumbs(true); .... That would start multiple threads, each thread with a new ThumbsLoader instance. They potentially all work on the same image in parallel. Most of the loaders will stop after they processed one file.

An other corner case is a layer merge while thumbnails are loaded. The problem here is that thumbnails are switched off if the other layer is still loading thumbnails (thumbnail loading must be stopped for the other layer because we will lose control over the thumbnail loader of the other layer). That might be unexpected to the user. I have no idea how a running thumbnail loader can be migrated from one layer to another.

The patch originates from ticket:10854.