New front-end API draft

[olokobayusuf]

The goals of this will be:

• Support composite sharing payloads (multiple images, multiple videos, and so on) instead of restricting to one image or one video or one piece of text
• Finally take care of #44
• A more object-oriented front-end. This has been a recent trend across all the Nat* API's; NatCam, NatCorder, and NatMic have all moved away from the static class pattern to functional instances.

Here's a draft:

namespace NatShare { // No more `NatShareU`

    public class SharePayload : IDisposable? { // Does this need to be disposable?
        /// <summary>
        /// Subject of the share payload
        /// </summary>
        public string Subject { get; set; }
        /// <summary>
        /// Add plain text
        /// </summary>
        public void AddText (string text);
        /// <summary>
        /// Add an image
        /// </summary>
        public void AddImage (Texture2D image);
        /// <summary>
        /// Add media with at a given URI
        /// </summary>
        public void AddMedia (string uri);
        /// <summary>
        /// Commit the payload to the operating system for sharing
        /// </summary>
        public void Commit (Action completionHandler = null);
        /// <summary>
        /// Dispose the payload
        /// </summary>
        public void Dispose ();
    }

    public class MediaPayload : IDisposable {
        /// <summary>
        /// Create a media payload for a given image
        /// </summary>
        public MediaPayload (Texture2D image);
        /// <summary>
        /// Create a media payload for media at a given URI
        /// </summary>
        public MediaPayload (string uri);
        /// <summary>
        /// Get an image from the backing media
        /// </summary>
        public Texture2D GetImage (float time = 0f);
        /// <summary>
        /// Save the media to the camera roll
        /// </summary>
        public void SaveToCameraRoll (string album = null);
        /// <summary>
        /// Dispose the media payload
        /// </summary>
        public void Dispose ();
    }
}

[olokobayusuf]

See #49