Input handler for actions

[nezvers]

An option to bind more than one input to a single input action. For example Godot input actions and Unity's new InputSystem.
In Godot, there are several ways how to interact with actions, all of which take into account the deadzone (default 0.5).
ref: Input | InputMap
Input.is_action_pressed - true if any associated input is held
Input.is_action_just_pressed / released - true only for one frame
Input.get_action_strength - 0.0 to 1.0 for analogue input and 0 or 1 for other
Input.get_axis( negative_action, positive_action ) - gives result (example right_action minus left_action)

For more naming examples/influence:
Gamemaker:
key_check - true if the button is held
key_check_pressed / key_check_released - true for one frame

Unity:
GetKey - true if a key is held
GetKeyDown / GetKeyUp - true for one frame

[Bombfuse]

Added some really basic bindings in via #189, I think we still need to add the analogue stick related APIs.