lib.plot: Use pens/brushes to customize PolyXXX classes

[dougthor42]

(This issue was created because PR #117 could not be finished in its entirety)

All of the PolyXXX classes (PolyBars, PolyHistogram, etc.) in lib.plot currently set individual items like edgecolour, fillcolour, and fillstyle.

Instead, it would make for a much cleaner API to simply set Pens for lines and Brushes for fills.

Example:

# before:
hist_variable_binsize = wxplot.PolyHistogram(
    [2, 3, 4, 6, 3],
    [18, 19, 22, 25, 26, 27],
    fillcolour=wx.BLUE,
    edgewidth=2,
    edgecolour=wx.CYAN,
)

#after:
hist_variable_binsize = wxplot.PolyHistogram(
    [2, 3, 4, 6, 3],
    [18, 19, 22, 25, 26, 27],
    fill=Brush(wx.BLUE, style=BRUSHSTYLE_SOLID),
    outline=Pen(wx.CYAN, width=2, style=PENSTYLE_SOLID),    # or "edge=..."
)