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)
* PluginManager.registerFileType has a new property `addToScene` which allows you to inject the new file type into the LoaderPlugin of the given Scene. You could use this to add the file type into the Scene in which it was loaded.
* PluginManager.install has a new property `mapping`. This allows you to give a Global Plugin a property key, so that it is automatically injected into any Scenes as a Scene level instance. This allows you to have a single global plugin running in the PluginManager, that is injected into every Scene automatically.
* PluginManager.createEntry is a new private method to create a plugin entry and return it. This avoids code duplication in several other methods, which now use this instead.
* Game has a new property `hasFocus` which is a read-only boolean that lets you know if the window the game is embedded in (including in an iframe) currently has focus or not.
* Game.Config has a new property `autoFocus`, which is `true` by default, and will automatically call `window.focus()` when the game starts.
* Clicking on the canvas will automatically call `window.focus`. This means in games that use keyboard controls if you tab or click away from the game, then click back on it again, the keys will carry on working (where-as before they would remain unfocused)
