Add custom Finicky app icon path to config

[wcoder]

Describe the solution you'd like

• add an additional option to config, that can change app icon by icon path
• in-app use something like NSApplication.SharedApplication.ApplicationIconImage = <image from path>

Can resolve https://github.com/johnste/finicky/issues/186 for those who need.

[ema07cqc]

Up to this request! Great great utility anyway.