Related to #8734

Based on my understanding of reading of various Apple docs, we can offload a lot of our icon loading to another thread:

• NSWorkspace functions are thread-safe (as long its from one thread)
• applicationIconImage must be from the main actor
• NSImage is Sendable

We should offload the logic of setting up our custom icon onto a separate thread. Right now it causes a "hitch" in our app.