* 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
These were broken-and-disabled in a previous PR because of 2 things:
- The Widgets' default vertical scrollbar size is now 2: this has to be reflected in these tests, in order to benefit from their "regressions spotting" role
- As long as textual#581 ("Default versus User CSS") is not solved, the Widget's default CSS overrides the custom Widgets' own CSS.
The test now temporarily uses a more specific CSS selector in order to bypass this.
