Ordering of import thumbnails

[XXLRay]

I presented this idea for a new feature before in the forum ( http://openshotusers.com/forum/viewtopic.php?f=27&t=1561 ) and was asked to open a bug report.

Currently Imported files are sorted by import date. You can order them by filename, length or label. My problem is, that some of my projects grow bit by bit. To navigate my imports comfortably I would need to be able to reorder the thumbs e.g. by right-clicking them and choosing "move up" or "move down" from the drop-down menu. In principle I could give them a number as label but this is very inconvenient doing it for more than 100 thumbs. It would be different if you gave the files a number as label by default.