Add Position struct + Radial API changes

[mgierlasinski]

PR changes:

• add Position struct (Point + value type)
• remove compiler warnings from Offset and Dimensions by implementing Equals/GetHashCode
• Radial gradient - use Position for Center, replace RadiusX and RadiusY with single Radius, remove Flags (absolute/proportional are now handled by Position/Dimensions constructed from Offset)
• add Unit Tests project for Core, import relevant tests from MG 1.*
• small API changes