Add macOS specific tray icon

[bertfrees]

It's a slightly simplified and monochromatic version of the known Pipeline logo. The image file is named according to the standard that Electron expects, so that macOS magically inverts the image (white on blue instead of black on white) when clicked, and so that the dimensions are right.

I've only included a high density (@2x) image and tested this only on my Macbook with retina display. I fear that it might not work with a normal display.

The resolution of the image is 40x40. The recommended size is 32x32, but I've made it a little bigger so that dimensions of the globe are closer to 32x32 and the pipes stick out a little. This makes it fit better with icons of other apps.

I recreated the original logo with SVG and converted to PNG using GIMP.

Electron documentation: https://www.electronjs.org/docs/latest/api/tray

[marisademeglio]

I just tested this PR and I think it looks fine on my external LG display as well as my macbook retina.

from the external display: