Support CornerRadius in Clock control

[corvinsz]

The Clock exposes a CornerRadius property but does not actually use it in it's template.

The converter FixedBottomCornerRadiusConverter is there to apply the CornerRadius to only the top left & top right corners of the "header":

[nicolaihenriksen]

LGTM. Perhaps consider an explicit default value for the CornerRadius in the (base) style? If I am not mistaken, the old code would use "2", where the new code will use "0" unless you explicitly set it to something else?

[corvinsz]

@nicolaihenriksen you are indeed correct, thanks for the heads up.
For some reasons a few styles already had a CornerRadius of 8 applied to them. I just left them as is.