Change color Offset from double to struct

[mgierlasinski]

Change Offset property from double to ColorOffset structure:

public struct ColorOffset
{
   public OffsetUnit { get; set; }
   public double Value { get; set; }
}

public enum OffsetUnit 
{
   Proportion, Pixels, Percent
}

With double type there is no way to distinguish between percent and pixels when CSS is parsed:
1px => 1 (valid proportion and pixel value)

Provide Offset type converter for XAML:

• Offset="0.25" => no changes
• Offset="25%" => calculate to 0.25
• Offset="34px" => calculate based on canvas size