py,st3m: ViewManager: Add is_active(view) method

Makes it easy for views to know whether they're still the active
one, so unwanted activity can be skipped when transitioning out.

python_payload/st3m/ui/view.py

@@ -258,3 +258,9 @@ class ViewManager(Responder):
             return self._incoming.show_icons()
         else:
             return True
+
+    def is_active(self, view: View) -> bool:
+        """
+        Returns true if the passed view is currently the active one.
+        """
+        return self._incoming == view