Create a thumbnailer for .akira files

## Expected Behavior
Akira files should show a thumbnail on file browsers of the available file.

## Possible Solution
I once played with the idea of having Spice-Up show thumbnails of the file. I had it working on the Gnome File browser but it was not working on the elementary browser. But basically we just need to make the Application class have a cli arg that creates a thumbnail from the file. It's simpler if we can store a thumbnail inside the .akira zipped file while saving and just use that instead.

https://github.com/Philip-Scott/Spice-up/pull/121