Add the possibility to register global input handlers

[miroiu]

📝 Description of the Change

Adds the possibility to register global input handlers via InputProcessor.Shared<T>. Once the desired input handlers are registered, an instance of the shared handler must be added to the InputProcessor instance of a UI element.

Example:

static MainWindow()
{
    // register a handler factory in a static context
    InputProcessor.Shared<Connector>.RegisterHandlerFactory(elem => new ConnectorState.Connecting(elem));

    // removes the handler factory for the specified handler type
    InputProcessor.Shared<Connector>.RemoveHandlerFactory<ConnectorState.Connecting>();
}

public Connector()
{
    // use shared handlers for this connector
    InputProcessor.AddSharedHandlers(this);
}

The handlers are only constructed once per element, when creating an instance of the shared class (e.g. new InputProcessor.Shared<Connector>(this) or using the InputProcessor.AddSharedHandlers extension method).

Important

Calling InputProcessor.Shared.RegisterHandlerFactory will not add new handlers to existing shared instances.

Methods:

• RegisterHandlerFactory
• RemoveHandlerFactory
• ClearHandlerFactories

Related #146

🐛 Possible Drawbacks

None.