Add monochrome icons

[ghost]

• Added monochrome icons for macOS
• Icons are available in SVG and PNG (32 X 32) format. Bigger PNG can also be generated using the provided SVG icons
• Icons are available for 0, 20, 40, 60, 80 and 100 percent.

[tom-james-watson]

Do you have any screenshots of these rendered in the mac tray? From what I remember the tray icons on macOS require a very specific format / colouring. At a glance at the pngs though I'm not sure about them aesthetically to be honest. Especially the 100 icon doesn't look great. It's also a different image to the logo, whereas for example the icons in https://github.com/tom-james-watson/breaktimer-app/pull/108/files are using the actual logo.

I've actually thought about just changing the logo/icon completely - the current one is one I made years ago and to be honest is a bit crap lol. I was thinking about making a new one using an hour glass design.

[ghost]

Ohh, I used the SVG from this PR as a base. Also the colors would get inverted. But I agree they look a little different.

Anyways, no worries. If you are planning to change the icon, just create the different monochrome icons if possible, or keep the SVG image in the repo. Then someone else can also pick up the task.

[tom-james-watson]

Yeah I think we can just sideline this for now and come back and do it properly with a nicer icon. Thanks anyway!