Add ability to control key background and outline in Theme

[tlnagy]

If placing the key inside a plot, it would be nice to be able to set the key background color and outline. For example, for this plot style, it would be to have a white background and black outline for the key too.

[Mattriks]

The way to do this would be similar to #1458,. Make a new struct like:

struct BoxFormat
    fill
    stroke
    linewidth
    etc
end

which could be accepted by new Theme fields e.g. Theme(panelbox=boxformat(...), keybox=boxformat(...)). For panel and background, the old Theme fields (panel_fill, panel_stroke etc) can be deprecated.

[tlnagy]

I think that's a good idea. Going forward, I think Theme would benefit from a hierarchical construction which also allows sharing of subthemes in a CSS-like manner.