Cells + widgets + extensions

[edgarfgp]

This PR adds support for Cell based on https://docs.microsoft.com/en-us/xamarin/xamarin-forms/user-interface/controls/cells

• TextCell
  • ImageCell (derives from ITextCell so it has access to the text extension . This is per spec)
  • SwitchCell
  • EntryCell (Added a custom control so we can add have TextChanged, like v1)
  • ViewCell