Radio button UC: provide minimal painting in designer mode

The circle relies on GDI+ and has no designer fallback, but with this
commit there's at least enough painting to visualize a control's
location.
Radio button UC: tie all color decisions into PD's central themer

This creates issues in designer mode, obviously, but a minimal
workaround has been provided.  (Just enough to enable positioning the
control.)

How comprehensive to make PD's color theming abilities is still a big
question in my mind, and I don't intend to settle it prior to 6.4's
release.  As I gradually tie various program elements into the central
themer, I'm keeping a running list of necessary color options.  (My gut
feeling is that PD will ultimately need only a small subset of colors,
but it's hard to know in advance.)

Any decisions on a final theme format will be contingent on PD's color
requirements, so for now, I'm just using a hard-coded list of colors
that covers only what is needed at present.  That list will continue to
expand until theming has been implemented program-wide, and at that
point, I should have a better idea of what's necessary.

Also in this commit, the now-unnecessary "ForeColor" property has been
removed from the UC.