Remove the hacky `value` field and `from_value` method of axis-like inputs in favor of a new input mocking API

[Shute052]

• should be merged once #514 was merged.
• added new methods for sending value changed events to each axis within the input:
  • fn send_axis_values(&self, input: UserInput, values: impl IntoIterator<Item = f32>).
  • fn send_axis_values_as_gamepad(&self, input: UserInput, values: impl IntoIterator<Item = f32>, gamepad: Option<Gamepad>).
• renamed MockInput::send_input method to MockInput::press_input to better reflect its current functionality for clarity.
• removed the hacky value field and from_value method from SingleAxis and DualAxis, in favor of new input mocking API.

Migration Guide

If you were using the send_input method for buttons and keys to send simulated pressed events, simply update the method name to press_input.

If you were using the value field or the from_value method to create a SingleAxis or DualAxis instance for sending simulated input events, you can remove them entirely. Simply use a normal version of these inputs and call the send_axis_values method with your desired values.

[alice-i-cecile]

Oh thank goodness I hated that design!

[alice-i-cecile]

Broadly in favor of the refactor: I like how this looks, and the split is cleaner. The diff is quite messy still: can you rebase?