Implement lazy loading of thumbnails

[buzzingwires]

Now for one of the more ambitious changes discussed in #651 The general goal is to allow thumbnails to be displayed in a lazy-loaded manner, and unloaded when they fall out of a certain range. This can save memory and improve startup times at the cost of potentially higher disk IO and CPU usage when browsing thumbnails.

The number of thumbnails displayed at any given time is configurable to this end by the following options. The default values should maintain the original behavior.

• thumbnail.load_behind : Max thumbnails to render behind the selected one. -1 (default) is no limit
• thumbnail.load_ahead : Max thumbnails to render ahead of the selected one. -1 (default) is no limit
• thumbnail.unload_threshold : Don't unload thumbnails unless more are loaded than this. -1 (default) is no limit