OpenRaster .ora support

[Qronikarz]

I wanted to just open it in the thumbnailer repository, but it would be also nice if Mint's image viewer allowed to view these pictures.

OpenRaster is a file format that allows to save multiple image layers so they are not lost. It's certainly not popular, but useful (https://en.wikipedia.org/wiki/OpenRaster)

Here an example OpenRaster image and how it looks on Mint desktop:

Now what's interesting in my opinion is that you can open these files in archiver (even the default Mint one) and there are two files that the thumbnailer and image viewer could use:

mergedimage.png could be displayed in the image viewer since it shows image with all layers merged. And the Thumbnails folder includes thumbnail.png which maybe could be used by the Mint's thumbnailer.