Add a Label widget

For the moment this does nothing more than inherit from a Static; but what
it does do is make it easier for someone to add text to their application
and to style it by styling all the Labels. Before now it would be common to
use a Static but if you try and style (or query) all Statics, you'd also get
things like Buttons, which inherit from Static.

See #1190

examples/five_by_five.py

@@ -13,7 +13,7 @@ from textual.containers import Horizontal
 from textual.app import App, ComposeResult
 from textual.screen import Screen
 from textual.widget import Widget
-from textual.widgets import Footer, Button, Static
+from textual.widgets import Footer, Button, Label
 from textual.css.query import DOMQuery
 from textual.reactive import reactive
 from textual.binding import Binding
@@ -33,10 +33,10 @@ class Help(Screen):
         Returns:
             ComposeResult: The result of composing the help screen.
         """
-        yield Static(Markdown(Path(__file__).with_suffix(".md").read_text()))
+        yield Label(Markdown(Path(__file__).with_suffix(".md").read_text()))
 
 
-class WinnerMessage(Static):
+class WinnerMessage(Label):
     """Widget to tell the user they have won."""
 
     MIN_MOVES: Final = 14
@@ -91,9 +91,9 @@ class GameHeader(Widget):
             ComposeResult: The result of composing the game header.
         """
         yield Horizontal(
-            Static(self.app.title, id="app-title"),
-            Static(id="moves"),
-            Static(id="progress"),
+            Label(self.app.title, id="app-title"),
+            Label(id="moves"),
+            Label(id="progress"),
         )
 
     def watch_moves(self, moves: int):
@@ -102,7 +102,7 @@ class GameHeader(Widget):
         Args:
             moves (int): The number of moves made.
         """
-        self.query_one("#moves", Static).update(f"Moves: {moves}")
+        self.query_one("#moves", Label).update(f"Moves: {moves}")
 
     def watch_filled(self, filled: int):
         """Watch the on-count reactive and update when it changes.
@@ -110,7 +110,7 @@ class GameHeader(Widget):
         Args:
             filled (int): The number of cells that are currently on.
         """
-        self.query_one("#progress", Static).update(f"Filled: {filled}")
+        self.query_one("#progress", Label).update(f"Filled: {filled}")
 
 
 class GameCell(Button):