Thumbnails improvements

[livanh]

Hi, I made some small changes involving thumbnails:

• allow thumbnails for directories
• when icon size is smaller than a configurable threshold, no thumbnail is shown, only icon (I think very small thumbnails are not useful anyway). This is controlled by [ui]/thumbnail_threshold=XX in libfm.conf
• when using thumbnails, also show the original file icons on top of them, reduced to half the size and placed in a corner. This is controlled by [ui]/thumbnail_overlay=<0|1> in libfm.conf. See screenshot.
  

[LStranger]

Thank you very much. Will consider it for version 1.4.0.