Add screen_(sub_)title properties to header.

Related review comment: https://github.com/Textualize/textual/pull/3199/files#r1321226453.

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   - `Screen.SUB_TITLE`
   - `Screen.title`
   - `Screen.sub_title`
+- Properties `Header.screen_title` and `Header.screen_sub_title` https://github.com/Textualize/textual/pull/3199
 
 ### Changed
 

src/textual/widgets/_header.py

@@ -160,18 +160,26 @@ class Header(Widget):
     def _on_click(self):
         self.toggle_class("-tall")
 
+    @property
+    def screen_title(self) -> str:
+        screen_title = self.screen.title
+        title = screen_title if screen_title is not None else self.app.title
+        return title
+
+    @property
+    def screen_sub_title(self) -> str:
+        screen_sub_title = self.screen.sub_title
+        sub_title = (
+            screen_sub_title if screen_sub_title is not None else self.app.sub_title
+        )
+        return sub_title
+
     def _on_mount(self, _: Mount) -> None:
         def set_title() -> None:
-            screen_title = self.screen.title
-            title = screen_title if screen_title is not None else self.app.title
-            self.query_one(HeaderTitle).text = title
+            self.query_one(HeaderTitle).text = self.screen_title
 
         def set_sub_title(sub_title: str) -> None:
-            screen_sub_title = self.screen.sub_title
-            sub_title = (
-                screen_sub_title if screen_sub_title is not None else self.app.sub_title
-            )
-            self.query_one(HeaderTitle).sub_text = sub_title
+            self.query_one(HeaderTitle).sub_text = self.screen_sub_title
 
         self.watch(self.app, "title", set_title)
         self.watch(self.app, "sub_title", set_sub_title)