Improve window cursor usability

[lewiszlw]

Objective

• Let user can use pattern like below when creating window

...
cursor: Cursor {
    hit_test: true,
    ..default()
},
...

• make hit_test default false

[james7132]

The physical_position field was deliberately made private in #7297. This seems like a regression there.

[lewiszlw]

If not set physical_position field public, users can't create Cursor by themselves. I thought CursorBuilder will be a bit heavy, but the solution depends on you.

[lewiszlw]

Maybe we can add comment to warn users not modify physical_position field directly.

[LiamGallagher737]

I think we should add a Cursor::new(grab_mode, visible, hit_test, icon) method, currently if someone wants to modify the cursor of a window they have to construct it with Cursor::default() and then modify the field they want to change.

Example of someone experiencing this issue. https://discord.com/channels/691052431525675048/1083143550738386984

[lewiszlw]

Closing in favor of https://github.com/bevyengine/bevy/pull/7988.