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/about.rs

@@ -13,6 +13,7 @@ impl super::Demo for About {
             .default_width(320.0)
             .default_height(480.0)
             .open(open)
+            .resizable([true, false])
             .show(ctx, |ui| {
                 use super::View as _;
                 self.ui(ui);