Register callbacks at message pump level, invoke them after refresh (#607)

* Register callbacks at message pump level, invoke them after refresh

* Fix a typo

* Code review feedback actions

* call_later callbacks invoked after refresh or on idle

* Fix space key in text input

* Make Widget.on_idle synchronous

* Fix call_later

* Rename PostScreenUpdate to InvokeCallbacks, and only fire if callbacks exist

* Update type hints for InvokeLater callbacks

* Update type signature of call_later callbacks, extract typevar