The `KeyboardManager` class has been removed. It has been replaced with `KeyboardPlugin` which is now an Input level plugin, that registers itself with the new `InputPluginCache`. The Input Plugin class (which belongs to a Scene) will now automatically inject registered plugins into itself on boot. Every Scene has its own instance of the Input Plugin (if enabled in the scene plugins), which in turn has its own instance of the KeyboardPlugin. The `InputManager` no longer has any reference to the Keyboard class at all. The benefits of this are two-fold: First, it allows you to now entirely exclude all of the keyboard classes from a custom build, saving a lot of space if not required. Secondly, it means that the Scenes themselves are now responsible for keyboard events, where-as before they were entirely global. This means a Scene can be paused and stop processing keyboard events, and stop having its Key objects updated, while another Scene can still carry on doing this. It also prevents key related callbacks in sleeping Scenes from being fired (which resolves issue #3733, thanks @JoeMoov2)
Every Plugin has been updated to correctly follow the same flow through the Scene lifecycle. Instead of listening for the Scene 'boot' event, which is only dispatched once (when the Scene is first created), they will now listen for the Scene 'start' event, which occurs every time the Scene is started. All plugins now consistently follow the same Shutdown and Destroy patterns too, meaning they tidy-up after themselves on a shutdown, not just a destroy. Overall, this change means that there should be less issues when returning to previously closed Scenes, as the plugins will restart themselves properly.
If the global top only flag is on and a scene consumes an input event then they won't flow any further down the scene list. This is optional (but on by default), allowing you to now correctly create a UI Scene above a game scene without the input events polluting one to the other.
