Allow setting an app icon when running from a standalone LUAZIP app

[rdw-software]

This issue is only for Windows since I'm not sure how important this is for macOS, and it's a huge mess on Linux. When running a standalone app, it currently uses a "placeholder"-looking default icon. It's possible to set icons for GLFW windows and also for WebViews, but I'm not sure if that's enough? In theory, if running without a console window (#537) only the window icon matters?

Once LUAZIP apps can be spawned without an attached console window, should revisit this and decide if more needs to be done.