API: Define API for pandas plotting backends
In #26414 we splitted the pandas plotting module into a general plotting framework able to call different backends and the current matplotlib backends. The idea is that other backends can be implemented in a simpler way, and be used with a common API by pandas users.

The API defined by the current matplotlib backend includes the objects listed next, but this API can probably be simplified. Here is the list with questions/proposals:

Non-controversial methods to keep in the API (They provide the `Series.plot(kind='line')`... functionality):
- LinePlot
- BarPlot
- BarhPlot
- HistPlot
- BoxPlot
- KdePlot
- AreaPlot
- PiePlot
- ScatterPlot
- HexBinPlot

Plotting functions provided in pandas (e.g. `pandas.plotting.andrews_curves(df)`)
- andrews_curves
- autocorrelation_plot
- bootstrap_plot
- lag_plot
- parallel_coordinates
- radviz
- scatter_matrix
- table

Should those be part of the API and other backends should also implement them? Would it make sense to convert to the format `.plot` (e.g. `DataFrame.plot(kind='autocorrelation')`...)? Does it make sense to keep out of the API, or move to a third-party module?

Redundant methods that can possibly be removed:
- hist_series
- hist_frame
- boxplot
- boxplot_frame
- boxplot_frame_groupby

In the case of `boxplot`, we currently have several ways of generating a plot (calling mainly the same code):
1. `DataFrame.plot.boxplot()`
2. `DataFrame.plot(kind='box')`
3. `DataFrame.boxplot()`
4. `pandas.plotting.boxplot(df)`

Personally, I'd deprecate number 4, and for number 3, deprecate or at least not require a separate `boxplot_frame` method in the backend, but try to reuse `BoxPlot` (for number 3 comments, same applies to `hist`).

For `boxplot_frame_groupby`, didn't check in detail, but not sure if `BoxPlot` could be reused for this?

Functions to register converters:
- register
- deregister

Do those make sense for other backends? 

Deprecated in pandas 0.23, to be removed:
- tsplot

To see what each of these functions do in practise, it may be useful this notebook by @liirusuk: https://github.com/python-sprints/pandas_plotting_library/blob/master/AllPlottingExamples.ipynb

CC: @pandas-dev/pandas-core @tacaswell, @jakevdp, @philippjfr, @PatrikHlobil