Feature: Image overlay tool to add reference images onto the map

[Senzaiken]

New Image Overlay tool to allow the additional of reference images into the map layer, either above or below.

Supported image types are Jpg, PNG and BMP. Loaded images persist through restarts.

• Reference images can be placed either above or below the land map.
• Customize position of the reference images' top left corner to any X/Y coordinate.
• Scale images (Default is ~1 px per tile)
• Opacity slider to allow the images to be made transparent for easier view of the ground beneath.
• Screen slider to only remove darker/black background images to allow line art to easily be seen.