BUG: fix Panel.fillna() ignoring axis parameter

This fixes #8251. It may need some more comprehensive tests but before I go too much farther I want to make sure the axis numbering scheme makes sense . The default behavior of `fillna` is to fill along `axis=0`. In the case of a DataFrame this fills along the columns. However, in the case of a Panel, filling along `axis=0` means you're filling along items. I'm not sure if that's the behavior that most users would think is the default. You might note that I also removed the references to what the behavior of each axis value is in the docstring since it gets a bit arbitrary when you consider both DataFrames and Panels.
It looks like `Panel.apply` passes series from its constituent dataframes rather than the dataframes themselves. That could still work, but the function would have to reconstruct the panel from series rather than dataframes. I'm not quite sure how I would go about doing that at the moment, but I'll pursue it if that's your preferred solution.

As an aside, I'm a little surprised at the behavior of `Panel.apply`. `Series.apply` passes data points, `DataFrame.apply` passes series, so by analogy I would expect `Panel.apply` to pass dataframes.