Add favicon for mac

[MadejaMaciej]

It has to have 512px width and height

[mrfatguy]

@MadejaMaciej It seems that you have made this 512x512 icon yourself. Keep in mind that we have all the assets prepared for all the platforms, based on what Electron guide specifies.

Make sure that you have it working, as you have used .png file where Electron guide requires you to use .icns file instead.

Reopening. Close again after you verify that this truly works on MacOS, even though you have used .png file. Thanks.

[MadejaMaciej]

It works, png vs icn doesn't matter from a long time. Compiler just where playing tricks on me and would not compile as for windows and linux I had to use 256px one, and for mac minimal one is 512px.