App icon fix

[Netkuszkusz]

Hi there,

As for now, the AppFlowy executable under Windows only have a 256x256 pixel image, therefore the EXE and it's shotcut appears pixellated and ugly nearly in any size (at least under Windows 10):

Can you please include proper 48, 32, 16 px images as well beside the 256 px one? This is the way. I've created these quickly:

Thanks!