Add support for retrieving device types per engine

[zmughal]

Something like list_devices. As these are going to be different for each
engine, perhaps standardizing info about the devices might be useful (e.g.,
using MIME types for files). Some of this information could be directly placed
in the upstream dists.

[mohawk2]

This is now implemented in 3d7f16f (PGPLOT, which always had the API available) and d674bea (PLplot, with API added by you).

[zmughal]

Right, I think I wanted to provide a devices to MIME types / file extension mapping as well because each engine uses different names. This can be done in a future API after the backend split. The idea behind this is that other code can inspect the devices available and choose from that given a requested filename. I believe this could be incorporated for Jupyter notebook support (right now I use P::G::Gnuplot directly in most of my notebooks).

One issue: there might be multiple devices that support the same output type (multiple PNG backends).

[mohawk2]

Do feel free to reopen this, if you can provide example code that the user would run. PGS is absolutely the right place to have that uniform-ised interface!