Change app icon background to accommodate Dark Mode

[BaksiLi]

This is a suggestion to improve the app icon.

macOS Mojave has a new feature called Dark Mode which applied an overall dark theme to OS interface. This has changed the appearance of Finder as well.

OctoMouse's app icon (OctoMouse/OctoMouse/Graphics/Icon.png) is a black mouse pattern on a transparent background, which is invisible under the Dark Mode as shown above.

Therefore I suggest that it should be changed to a white background, so that it works for both Light and Dark appearance of macOS.

Regards.