Add basic gamepad support for bevy_ui by emulating a mouse

[alice-i-cecile]

What problem does this solve or what need does it fill?

bevy_ui is, first and foremost a game UI library. However, it is currently very challenging to work with via a gamepad.

What solution would you like?

When used in menus, gamepads should:

• control the mouse cursor with either analog sticks or the d-pad
• click buttons (typically when A is pressed)

This requires:

• a configurable sensitivity that converts these gamepad movements into mouse motion
• a way to detect whether the KBM or gamepad is being used to drive the UI
• a way to control whether or not this behavior should be enabled

What alternative(s) have you considered?

You can work around this by mocking mouse movement and mouse button presses, tied to gamepad controls.

However this is non-obvious, frustrating, and not terribly robust.

Additional context

A full-fledged UI navigation system as laid out in bevyengine/rfcs#41 would be very helpful to supplement this, but is not required. Emulating mouse-like behavior is plenty.

[alice-i-cecile]

/// A resource that determines which input mode the player is using
enum InputMode {
    KBM,
    Gamepad,
}

fn set_input_mode(
    mut input_mode: ResMut<InputMode>,
    gamepad_events: EventReader<GamepadEvent>,
    keyboard_events: EventReader<KeyboardInput>,
    mouse_events: EventReader<MouseMotion>,
) {
    if !keyboard_events.is_empty() || !mouse_events.is_empty() {
        *input_mode = InputMode::KBM;
    }

    if !gamepad_events.is_empty() {
        *input_mode = InputMode::Gamepad;
    }
}

This code is working well to set the input mode for me.

[nicopap]

Note that RFC 41 is fully implemented in #5378 However, I'm still missing a story for mixing hover and focus highlights.

[alice-i-cecile]

Removing D-Complex, as I think that the very basic form of this is relatively straightforward to do.