Visualization: Callback, extensions etc.

Use this issue to track open questions/issues related to the new visualization approach of plotting directly from the REPL:

- [x] Document capabilities for 2D slices of 3D data (done in #403)
- [x] Continuing #350, it would be nice to have a callback supporting visualization on the fly while running a simulation (done in #403)
- [x] 1D plots (tracked in #198)
- [x] general plotting
- [x] plot from 2D/3D
- [x] Rewrite and consolidate current approach
- [ ] Plot on arbitrary points to, e.g., support plotting values along a semicirlce
- [ ] Support plotting for structured curved mesh
- [x] 2D
- [ ] 3D
- [x] Think about how to support Makie
- [ ] Reading solution/restart files for plotting
- [ ] Possibilities for batch processing of HDF5 files
- [ ] Extend to plotting of semidiscretization + coefficients (and document that part)
- [ ] Document how to access (and work with) uniformly spaced data from `PlotData2D` etc.
- [ ] How to handle the dependency on the Plots (and possibly other) package(s) (see also https://github.com/trixi-framework/Trixi.jl/pull/403#discussion_r549679762)?
- [ ] Optimize plotting performance (especially mesh plotting becomes unbearably slow when the number of leaf cells increases)
- [ ] Movie callbacks (https://github.com/trixi-framework/Trixi.jl/issues/842)
- [ ] Flexible `PlotDataX` axes (https://github.com/trixi-framework/Trixi.jl/issues/590)
- [ ] In-situ 3D plotting (https://github.com/trixi-framework/Trixi.jl/issues/859)