Fix typing issues in demo.

Fixing the typing issues here involved making use of the messaging features to replace a method (wasn't exactly wrong, but this is more in line with the Textual way of doing things, which should be prevalent in the demo of the framework).
We also dealt with a typing issue in DarkSwitch.on_dark_change by deleting an unnecessary parameter, but the underlying problem is unsolved and was logged as issue #1865.

Related issues: #1865.

mypy.txt

@@ -40,7 +40,4 @@ src/textual/widgets/_data_table.py:1629: error: Argument "key" to "sorted" has i
 src/textual/widgets/_button.py:219: error: Item "ConsoleRenderable" of "Union[ConsoleRenderable, RichCast, str]" has no attribute "copy"  [union-attr]
 src/textual/widgets/_button.py:219: error: Item "RichCast" of "Union[ConsoleRenderable, RichCast, str]" has no attribute "copy"  [union-attr]
 src/textual/widgets/_button.py:219: error: Item "str" of "Union[ConsoleRenderable, RichCast, str]" has no attribute "copy"  [union-attr]
-src/textual/demo.py:206: error: Argument 3 to "watch" of "DOMNode" has incompatible type "Callable[[bool], None]"; expected "Union[Callable[[], Awaitable[None]], Callable[[], None]]"  [arg-type]
-src/textual/demo.py:221: error: "App[Any]" has no attribute "add_note"  [attr-defined]
-src/textual/demo.py:276: error: "App[Any]" has no attribute "add_note"  [attr-defined]
-Found 44 errors in 9 files (checked 154 source files)
+Found 41 errors in 8 files (checked 154 source files)