Add the ability to open a window on a given screen

This is done by supplying the screen in the `WindowOptions` struct.
Note that it's optional, and we will let the operating system choose
which screen to show the window on when `screen` is not provided, as
we did before this change.

crates/gpui/src/platform/test.rs

@@ -131,8 +131,8 @@ impl super::Platform for Platform {
 
     fn quit(&self) {}
 
-    fn screen_size(&self) -> Vector2F {
-        vec2f(1024., 768.)
+    fn screens(&self) -> Vec<Rc<dyn crate::Screen>> {
+        Default::default()
     }
 
     fn open_window(