added implicit conversion between CrossGraphics.Color and System.Drawing.Color

Added conversion operators between our Color type and the standard one.
 I'm still not sure exactly why CrossGraphics defines its own Color
type, yet uses PointF and RectangleF from System.Drawing.  But at least
with these converters, the pain is minimal.
Tweaked the clock sample so that the thick border around the clock face fits within the window.

On Mac, at least, thick borders created by DrawOval (etc.) are centered
on the coordinates you specify.  So if you want them to fit in the
bounds of the window without being clipped, you have to inset those
coordinates by half the line width.  (If this isn't true on other
platforms CrossGraphics supports, then we should talk about whether/how
to make them all behave the same.)