Add a few missing equality operators to Input structs.

Add equality operators and Object.Equals() overrides to the
GamePadButtons, GamePadDPad, GamePadThumbSticks, and GamePadTriggers
structs in the MonoGame.Framework.Input namespace.
Merge pull request #2046 from PrecisionMojo/_pm/add-missing-input-equality

Add a few missing equality operators to Input structs.