Add window center properties (#2056)

* add window center properties

* oh right, docstrings

arcade/application.py

@@ -915,6 +915,21 @@ def on_mouse_leave(self, x: int, y: int):
         """
         pass
 
+    @property
+    def center(self) -> tuple[float, float]:
+        """Returns the coordinates of the center of the window."""
+        return (self.width / 2, self.height / 2)
+
+    @property
+    def center_x(self) -> float:
+        """Returns the X-coordinate of the center of the window."""
+        return self.width / 2
+
+    @property
+    def center_y(self) -> float:
+        """Returns the Y-coordinate of the center of the window."""
+        return self.height / 2
+
 
 def open_window(
     width: int,