Move duplicate safe calls to update_styles into a single method

It's done as an internal, but can be called from child classes of course.
This is intended to be a single central method of asking the app to update
styles while also not caring if there is no active app available just yet.

src/textual/dom.py

@@ -874,6 +874,16 @@ class DOMNode(MessagePump):
         else:
             self.remove_class(*class_names)
 
+    def _update_styles(self) -> None:
+        """Request an update of this node's styles.
+
+        Should be called whenever CSS classes / pseudo classes change.
+        """
+        try:
+            self.app.update_styles(self)
+        except NoActiveAppError:
+            pass
+
     def add_class(self, *class_names: str) -> None:
         """Add class names to this Node.
 
@@ -886,10 +896,7 @@ class DOMNode(MessagePump):
         self._classes.update(class_names)
         if old_classes == self._classes:
             return
-        try:
-            self.app.update_styles(self)
-        except NoActiveAppError:
-            pass
+        self._update_styles()
 
     def remove_class(self, *class_names: str) -> None:
         """Remove class names from this Node.
@@ -903,10 +910,7 @@ class DOMNode(MessagePump):
         self._classes.difference_update(class_names)
         if old_classes == self._classes:
             return
-        try:
-            self.app.update_styles(self)
-        except NoActiveAppError:
-            pass
+        self._update_styles()
 
     def toggle_class(self, *class_names: str) -> None:
         """Toggle class names on this Node.
@@ -920,10 +924,7 @@ class DOMNode(MessagePump):
         self._classes.symmetric_difference_update(class_names)
         if old_classes == self._classes:
             return
-        try:
-            self.app.update_styles(self)
-        except NoActiveAppError:
-            pass
+        self._update_styles()
 
     def has_pseudo_class(self, *class_names: str) -> bool:
         """Check for pseudo class (such as hover, focus etc)"""