Crashes if installed in a non-standard location

[eugenesvk]

https://github.com/unsecretised/rustcast/blob/72118536c10065fb5e0107e251706b9837abc115/src/app/menubar.rs#L60 hardcodes resources
"/Applications/Rustcast.app/Contents/Resources/icon.png"

which is a mistake since users can freely move the app around and install in other locations.
Instead it should reference the dynamic location of the app

[unsecretised]

Yeah this is something that I did not like doing... but couldn't think of an alternative at the point of time... I'll take a look

[Mnem42]

There are icons bundled into the binary in the windows-support branch (which is actually closer in role to a dev branch), it could just use those

[unsecretised]

Ok this is a simple fix. Will work on this.