Make CursorGrabMode a State

[Atlas16A]

Objective

Keeping various systems from running when the window isn't viewed/holding the mouse such as player movement requires needless extra steps.

Solution

Makes it so that users can use cursor grab mode as a state to control system runs.

[github-actions]

Welcome, new contributor!

Please make sure you've read our contributing guide and we look forward to reviewing your pull request shortly ✨

[mockersf]

It's possible to have several windows with different CursorGrabMode. What would the state mean in this case?

I would rather not have this in Bevy, and it's easy enough for an application that needs this to newtype CursorGrabMode and handle it as a state on its own.

[rparrett]

A couple other potential issues:

• Users would need to synchronize their window's state with this state. This seems awkward.
• This is a three-variant enum where you most likely only care about two variants, but need to deal with all three if you're managing the cursor in a cross-platform way.

You can probably get your desired behavior with a run condition.

app.add_systems(Update, some_system.run_if(cursor_grabbed));

fn cursor_grabbed(windows: Query<&Window, With<PrimaryWindow>>) -> bool {
    for window in &windows {
        return window.cursor_options.grab_mode != CursorGrabMode::None;
    }
    false
}