Started removing the usage of libspectre.

Reasoning:
* right now we're asking the backend to produce an EPS file for the plot and convert it via libspectre to PNG to render. We can directly ask the backend to generate the PNG file and remove this unneeded conversion.
* libspectre is very unstable and crashing often, at least on openSuse
* this library doesn't seem to be heavily developed/supported anymore
* later we'll switch to the PDF output in the backend systems and render it via poppler to PNG as it is already done for Jupyter content.

We start removing it for maxima now, other parts of the code will be addressed step by step.