Print preview displays inside progress ring

[tusing]

Describe the bug

The print preview displays inside the progress ring instead of replacing it, which looks fairly ugly (as the preview is square and the ring is circular). The preview should replace the ring altogether, or the progress ring should instead become a progress bar and display under the preview image.

To Reproduce

Enable print preview for your Prusa printer: add thumbnails = 16x16,256x256 to your config.ini.

Expected behavior

As shown in the image in the README, the preview should replace the ring as shown here:

Screenshots

General Information:

• Prusa MK3S
• Raspberry Pi 4 w/ Hyperpixel 4 (800x480) running Raspbian

[UnchartedBull]

I'll merge this with #749 to clean up the issues a litte. Good idea to have a normal progress bar so the preview could be a bit bigger, will be included in one of the next releases.