A Window can now be resizable in only one direction (#4155)

For instance: `Window::new(…).resizable([true, false])` is a window that
is only resizable in the horizontal direction.

This PR also removes a hack added in
https://github.com/emilk/egui/pull/3039 which is no longer needed since
https://github.com/emilk/egui/pull/4026

crates/egui_demo_lib/src/demo/code_example.rs

@@ -75,6 +75,7 @@ impl super::Demo for CodeExample {
             .default_size([800.0, 400.0])
             .vscroll(false)
             .hscroll(true)
+            .resizable([true, false])
             .show(ctx, |ui| self.ui(ui));
     }
 }