Serialize window maximized state in WindowSettings (#5554)

A user of my Windows application reported a papercut where the
application restores its size on next load, but does not restore its
maximized state. This PR fixes that.

To test, I patched https://github.com/emilk/eframe_template to use my
local code since I knew that template saves/restores window data.
Testing methodology was to simply `cargo run`, maximize the application,
then close the application. `cargo run` again and the application should
start maximized.

Closes #1517.

* [x] I have followed the instructions in the PR template
* * This is mostly true, I had difficulties running `./scripts/check.sh`
for some reason. Possibly a bad Python version?

crates/egui-winit/src/lib.rs

@@ -1824,6 +1824,9 @@ pub fn apply_viewport_builder_to_window(
             let pos = PhysicalPosition::new(pixels_per_point * pos.x, pixels_per_point * pos.y);
             window.set_outer_position(pos);
         }
+        if let Some(maximized) = builder.maximized {
+            window.set_maximized(maximized);
+        }
     }
 }