Add missing copy method for FieldSection

Also provide some deferring implementations for speedup.

Author: FichteFoll

ass/section.py

@@ -92,6 +92,9 @@ def set_data(self, fields):
             raise ValueError("Fields must be a mutable mapping")
         self._fields = fields
 
+    def __contains__(self, key):
+        return key in self._fields
+
     def __getitem__(self, key):
         return self._fields[key]
 
@@ -110,6 +113,12 @@ def __len__(self):
     def __repr__(self):
         return "{}({!r}, {!r})".format(self.__class__.__name__, self.name, self._fields)
 
+    def clear(self):  # Optional, but should be faster this way
+        return self._fields.clear()
+
+    def copy(self):
+        return self.__class__(self.name, self._fields.copy())
+
 
 class EventsSection(LineSection):
     field_order = Dialogue.DEFAULT_FIELD_ORDER