[FR] Use Preferences.jl to configure default loaded backend

[mkitti]

Loading GR by default can cause issues since it loads so many other packages, notably JLLs.

For example, loading QT via GR can result in the PyPlot backend not working:
https://discourse.julialang.org/t/trouble-using-plots-on-linux/90061/32?u=mkitti

To simplify this, let's provide a Preferences.jl mechanism so that users can load their preferred backend by default. This will also help to minimize issues and accelerate plots by not loading unnecessary packages that we are not using.

[mkitti]

The Discourse post got moved. See https://discourse.julialang.org/t/trouble-using-plots-on-macos/90182/19?u=mkitti

[t-bltg]

This will also also improve ttfp for the chosen default backend.
If a user only uses a declared backend, we should precompile examples for that backend.

[mkitti]

My main goal here is compatibility and usability. We're loading a lot of libraries with some of these plotting packages.

Performance is a secondary goal, but a worthy one to consider. Perhaps besides just indicating the default backend, we also indicate which backends to load by default in general?

[t-bltg]

This is going to be a tough one, because of the constraints of module initialization in __init__, precompilation using SnoopPrecompile and the way we load backends (currently using @eval and @require) in Plots ...

I tried some things, but going nowhere atm.