Enhance Focus event with from_app_focus argument

Added a new argument `from_app_focus` to the Focus event class to indicate whether the focus event was triggered by the application regaining focus or by user interaction within the Textual app. Updated the constructor to initialize this new attribute.

src/textual/events.py

@@ -16,10 +16,10 @@
 from dataclasses import dataclass
 from pathlib import Path
 from typing import TYPE_CHECKING, Type, TypeVar
-from typing_extensions import Self
 
 import rich.repr
 from rich.style import Style
+from typing_extensions import Self
 
 from textual._types import CallbackType
 from textual.geometry import Offset, Size
@@ -722,8 +722,18 @@ class Focus(Event, bubble=False):
 
     - [ ] Bubbles
     - [ ] Verbose
+
+    Args:
+        from_app_focus: True if this focus event has been sent because the app itself has
+            regained focus (via an AppFocus event). False if the focus came from within
+            the Textual app (e.g. via the user pressing tab or a programmatic setting
+            of the focused widget).
     """
 
+    def __init__(self, from_app_focus: bool = False) -> None:
+        self.from_app_focus = from_app_focus
+        super().__init__()
+
 
 class Blur(Event, bubble=False):
     """Sent when a widget is blurred (un-focussed).