Improvements over Image, Bitmap and Icon classes

[damiansalvia]

Several improvements over Image/Bitmap/Icon classes:

• Make Bitmap class inherit from Image class
• Base Image class on SKBitmap (instead of SKImage)
• Implement missing methods and properties
• Add PixelFormat, RotateFlipType, ImageLockMode, ImageFlags, GraphicUnit enums

[fedeazzato]

Magnificent work! 👏

I just added a couple of notes that I believe may need small improvements.

[damiansalvia]

I applied a fix for every suggestion except for the SvgImage proposal, I'm still considering the best way to implement it.

[damiansalvia]

I created an Svg class (sibling of Bitmap) to encapsulate the definition of SkiaSharp.Extended.Svg.SKSvg. This class also has a reference to XmlDocument for manipulating the SVG. Currently, it is used for transformations such as rotation and scaling provided by the RotateFlip method, but it could be useful for additional operations in the future.

This change also includes a refactor of the Image and Bitmap classes to remove the direct reference to SKBitmap in the Image class. Now, SKBitmap is only referenced by the Bitmap class.

[fedeazzato]

This work is awesome!

I just left a single question in Icon class.

[damiansalvia]

@fedeazzato Did you have time to check my replay on your comment? If you're agree, can we merge these changes into master?