Plotting a mesh without setting up a semidiscretization

It would be nice to be able to plot a mesh without setting up a semidiscretization first, so that something like
`plot(mesh)` would work, instead of having to use
```
pd = PlotData2D(sol)
plot!(getmesh(pd))
```

Start by tracing the existing PlotData2D(sol) and getmesh(pd) plotting path to find the mesh plotting entry point. The change is complete when plot(mesh) works without first creating a semidiscretization, with coverage for plotting a mesh directly.

Written by the indexing model from the issue text.