Fixed pos image

[boatwrong]

This PR adds a feature

• Insert image with fixed position #164

This is one by exposing positioning properties of the Image

        /// <summary>
        /// Get or set the Image's horizontal position.
        /// </summary>
        public HorizontalPosition horizontalPosition {
            get {
                if (_Image.Inline == null) {
                    var anchor = _Image.Anchor;
                    var hPosition = anchor.HorizontalPosition;
                    return hPosition;
                } else {
                    throw new InvalidOperationException("Inline images do not have HorizontalPosition property.");
                }
            }
            set {
                if (_Image.Inline == null) {
                    var anchor = _Image.Anchor;
                    anchor.HorizontalPosition = value;
                } else {
                    throw new InvalidOperationException("Inline images do not have HorizontalPosition property.");
                }
            }
        }

        /// <summary>
        /// Get or set the Image's vertical position.
        /// </summary>
        public VerticalPosition verticalPosition {
            get {
                if (_Image.Inline == null) {
                    var anchor = _Image.Anchor;
                    var vPosition = anchor.VerticalPosition;
                    return vPosition;
                } else {
                    throw new InvalidOperationException("Inline images do not have VerticalPosition property.");
                }
            }
            set {
                if (_Image.Inline == null) {
                    var anchor = _Image.Anchor;
                    anchor.VerticalPosition = value;
                } else {
                    throw new InvalidOperationException("Inline images do not have VerticalPosition property.");
                }
            }
        }

Includes an example and test case for the newly exposed properties.