Add JoystickAxisState to the project.

This class wraps the state of all joystick axes into a single object.
Fix angle units in the documentation of GamepadAxisState.
Add methods to retrieve Vector2 and JoystickAxisState objects to Joystick.
Add JoystickFilter to the project.

JoystickFilter represents a filter that can act on the axis values of
a Joystick.
Add JoystickFilterSet to the project.

JoystickFilterSet applies multiple filters to a Joystick.
Add SimpleJoystickFilter to the project.

SimpleJoystickFilter makes it easier to implement joystick filters.
Add SimpleGamepadJoystickFilter to the project.

This class makes it easier to implement filters that act both on
joysticks and on gamepads.
Make all existing filters act on both gamepads and joysticks.

The names of these filters were modified to reflect this change.
Add missing character to GamepadFilter documentation.