Print bed image for Artisan

[sillyfrog]

This adds the ability to add a background print bed image to all the plugins. I have included a quick image I have created for the Artisan (the source is available here: https://github.com/sillyfrog/snapmaker-bed ).

There maybe a better way to calculate the meshes_dir, but from a quick look at the Cura source code I couldn't figure it out.

This allows the addition of a print bed background image for when working in Cura, not only does it look nicer, especially for the Artisan, it allows the accurate placement of objects to only be on the "inner zone" of the heat bead when setting up the slicer.