Remember last preview color selection

[Nek-12]

I haven't found how to make the plugin remember the last preview background color selection globally. It always resets to transparency, but since our icons are all white, they are almost invisible on the checkerboard, and I have to clock black each time I open a file.

Additionally, the mode can be derived from the dominant icon color.

1. White or light 0-hue color -> Black bg
2. Black or dark 0-hue -> White bg
3. Any other -> checkerboard