Make Windows icons suitable for high dpi screens

The maximum size of the icons is now 256x256 pixels so that
Windows can correctly scale them.

The 256x256 layer for the main icon was created upscaling the
192x192 png available in the source tree. Hence, the icon might
not be perfect at the maximum size.

The icon for the file association was made from scratch and it's
visually similar to the previous icon.