Winit version 0.31.0-beta.2

Added

• Add EventLoopExtRegister::register_app for being explicit about how the event loop runs on Web.
• Add EventLoopExtNeverReturn::run_app_never_return for being explicit about how the event loop runs on iOS.

Changed

• On Web, avoid throwing an exception in EventLoop::run_app, instead preferring to return to the caller.
  This requires passing a 'static application to ensure that the application state will live as long as necessary.
• On Web, the event loop can now always be re-created once it has finished running.

Fixed

• Fixed panic when calling Window::set_ime_allowed.