Use Maui.Graphics primitive types

[mgierlasinski]

This PR aims to reduce dependency on Xamarin.Forms from MagicGradients.Core:

• Color, Point types from Xamarin.Forms replaced with Microsoft.Maui.Graphics
• Converters converted :hear_no_evil: to use System.ComponentModel instead of Xamarin.Forms
• Project MagicGradients.Graphics removed as MagicGradients.Core has now dependency on Microsoft.Maui.Graphics
• Xaml directory renamed to Converters to be more specific and follow convention of Maui structure