Add glfwSetWindowIcon

Adds support for setting window icons programmatically on platforms
where this makes sense.

Fixes #453.
Closes #467.

src/win32_platform.h

@@ -191,6 +191,8 @@ typedef VkBool32 (APIENTRY *PFN_vkGetPhysicalDeviceWin32PresentationSupportKHR)(
 typedef struct _GLFWwindowWin32
 {
     HWND                handle;
+    HICON               bigIcon;
+    HICON               smallIcon;
 
     GLFWbool            cursorTracked;
     GLFWbool            iconified;