Window: set window/app icon

[codymullins]

Problem

A desktop browser sets a window/app icon (taskbar, dock, alt-tab). Skyline has no icon API.

Current state

• No Icon on AppWindow or AppWindowOptions.
• Native APIs exist: glfwSetWindowIcon; macOS NSApplication.applicationIconImage.

What's needed

• An icon property/option accepting image data (e.g. PNG bytes or a small bitmap), settable at creation and at runtime.
• GLFW and AppKit implementations.

Acceptance criteria

• The configured icon appears in the dock/taskbar and alt-tab.