Added C#/Unity versions

This version is quite alike to the GameMaker one, consisting of 2 (3) static functions total:

• WindowIconTools.SetIcon(texture2d or null, kind)
  Changes small or big icon for the window.
  In non-Unity C#, you provide an array of bgra bytes, width, and height instead of a texture.
• WindowIconTools.SetOverlayIcon(texture2d or null, opt. description)
  Changes the overlay icon in the taskbar.
• WindowIconTools.SetProgress(progressState, numCompleted, numTotal)
  This is an equivalent of the other extension I made and is only here because it didn't really "cost" anything to add.

You can also peck at the sample project if you want - it has an example of using a render texture to continuously update the icon.

Things I learnt along the way:

• Unity stores texture pixels upside-down.
• You can't set a texture to import in BGRA format.
• Getting a handle of game window is a mess and always have been.
• Getting arguments or function order wrong for externs may not throw any error and is generally very confusing.