Add support for setting window icon

This adds a new property from Settings:🪟:iconand a Icon struct which can be converted to winit:🪟:Icon.

It also adds code to display this icon in Application::run. Due to the fact that the Icon struct is non copyable, I also had to remove the Copy trait from all Settings, both in `iced` and `iced_winit`.

src/settings.rs

@@ -2,7 +2,7 @@
 use crate::window;
 
 /// The settings of an application.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub struct Settings<Flags> {
     /// The window settings.
     ///