Add layout support in plot_frame similar to the hist_frame layout
Add an optional layout keyword argument in order to set the subplots layout. 
If none provided, falls back to default behavior.

- This needs a couple of tests which test passing an invalid layout.
- Pls add a mention in release notes (refereing this PR #)
 You will also have to expose this in `plot_frame` (https://github.com/pascal-schetelat/pandas/blob/master/pandas/tools/plotting.py#L1827) to make it usable in `df.plot()`
 @jreback I duplicated the same tests on the layout as in hist_frame in _get_layout. Do you think additional tests are required?

@jorisvandenbossche you mean adding an entry in the doc or explicitely passing a layout argument in plot_frame? 
 @pascal-schetelat Yes, at least adding it to the docs. 
I also meant to explicitly pass it as a argument, but I see that there are also others that are passed through the `*kwds`, so I am not really sure about this what is best.
 @pascal-schetelat this will close #6306, yes?
 @pascal-schetelat pls rebase and push again, your changes are not showing up here
 I think the shape of returned axes should be cared. `hist` returns ndarray which has the same shape as the specified layout, not flattened. Personally whichever possible if there is a policy.
