Exploring approaches to winit API

This is just a prototype showing my vision for the future winit API. The API itself is just rough idea on how it may look like

The design is motivated by endless amount of issues where we need to model feedback between the user and application and by the fact that GUI is generally single threaded.

The desired layout:

• winit-core - top-level traits not tied to any platform and ideally free from platform extensions.
• winit - glue around event loop creation in a cross platform way and managing extensions.
• winit-wayland - wayland backend implementing winit-core. Same should be done for other backends.
• winit-examples - examples for winit.

The example lives in the winit-wayland, but it'll be moved once the glue API is designed.