Refactor inputhook to allow easy extension.
Add module-level access to register method
Allow registering external event loops to integrate with kernel
Add docs about extending GUI integration
Deprecation warnings for enable_* functions in inputhook

Also, update examples to not use these
Add note about function deprecations in whatsnew doc
Merge pull request #5666 from takluyver/inputhook-extensible

Refactor inputhook to allow easy extension