This enables support for receiving and handling FocusIn and FocusOut
sequences, and turns then into AppFocus and AppBlur events.

While the terminal window didn't have focus, anything can could happen. The
widget could be removed, the screen could change, etc. So by the time
AppFocus happens the widget might not be one to focus any more.

Initially I was just making it the focused widget anyway and letting the
focus-handling code do what it needed to do. Sending focus to a widget that
isn't part of the DOM any more isn't exactly a breaking problem; but...

One issue is that you can end up with App.focused saying that a widget is
focused that isn't in the DOM any more. We don't want that. So here I'm a
bit more defensive. This changes things so that we check that the widget's
screen is still the screen that's in play. If the widget has been removed it
won't have a parent and so can't find its screen. All of this means that if
the screen has changed *or* if the widget has been removed, we're covered.

These tests don't test the actual act of blurring or focusing the
application (that's kind of hard to do in tests, really). What it does do is
test that widget focus does the right thing after each of those app-level
events.

Remove the text so say they're only for textual-web, but make it clear
textual-web is supported, as are any other terminals that support the
required sequences.

Co-authored-by: TomJGooding <101601846+TomJGooding@users.noreply.github.com>
Co-authored-by: Will McGugan <willmcgugan@gmail.com>

While the application is in an AppBlur state, it's possible that some code
could have been running that updated what's focused. It doesn't make sense
to have Textual itself override the dev's choice to have focus be somewhere
else (perhaps the result of some long-running background process, that
they've tabbed away from, and when they tab back they expect to be in a
specific control).

So here I tweak the code that restores the focused widget so that it only
restores if it's still the case that nothing has focus.