Adds the ability to save diagrams to a specified image file.
Features
- New Diagram.exportImage method saves a diagram to a specified image file.
- New Diagram.createImage method returns a UIImage containing the rendered diagram.
- Added new saveToXml and loadFromXml overloads that take the URL parameter.
- Added the ability to change background by setting a Diagram's brush or style.backBrush properties.
- Added support for Apple Swift 5.2.