Design for `Button` and `Signal`

[bvssvni]

• The field device is necessary to distinguish between devices with same button.
• The field key on Button is necessary to distinguish between buttons on same device.
• The field xyz on Signal is necessary to deal with 3D user input events.
• The field axis_id is necessary to distinguish between variants of buttons on the same device.
• The field user_id is necessary to distinguish between different users of a device.

Examples:

• A game pad might contain more than one axis controller, but it counts as one device, therefore it needs the axis_id field.
• A mixing table might contain more than one button, therefore it needs the axis_id field.
• A party game with multiple game pads needs multiple controllers, therefore it needs the user_id field.
• A robot arm contains several joints, therefore it needs the axis_id field.

[bvssvni]

Not used in favor of https://github.com/PistonDevelopers/input/pull/36

Closing.