Update no_echo metadata name to echo_update for clarity.

Having a negative in the name was confusing to me, so I renamed the property echo_update. Set it to False to disable echos for that attribute.

packages/schema/messages.md

@@ -316,7 +316,7 @@ The `echo_update` messages enable a frontend to optimistically update its widget
 
 Since the `echo_update` update messages are optional, and not all attribute updates may be echoed, it is important that only `echo_update` updates are ignored in the last step above, and `update` message updates are always applied.
 
-For attributes where sending back an `echo_update` is considered too expensive or unnecessary, we have implemented an opt-out mechanism in the ipywidgets package. A model trait can have the `no_echo` metadata attribute to flag that the kernel should not send an `echo_update` update for that attribute to the frontends. We suggest other implementations implement a similar opt-out mechanism.
+Implementation note: For attributes where sending back an `echo_update` is considered too expensive or unnecessary, we have implemented an opt-out mechanism in the ipywidgets package. A model trait can have the `echo_update` metadata attribute set to `False` to flag that the kernel should never send an `echo_update` update for that attribute to the frontends. Additionally, we have a system-wide flag to disable echoing for all attributes via the environment variable `JUPYTER_WIDGETS_ECHO`. For ipywdgets 7.7, we default `JUPYTER_WIDGETS_ECHO` to off (disabling all echo messages) and in ipywidgets 8.0 we default `JUPYTER_WIDGETS_ECHO` to on (enabling echo messages).
 
 #### State requests: `request_state`
 

python/ipywidgets/ipywidgets/widgets/tests/test_set_state.py

@@ -345,7 +345,7 @@ def _square(self, change):
 def test_no_echo():
     # in cases where values coming from the frontend are 'heavy', we might want to opt out
     class ValueWidget(Widget):
-        value = Float().tag(sync=True, no_echo=True)
+        value = Float().tag(sync=True, echo_update=False)
 
     widget = ValueWidget(value=1)
     assert widget.value == 1

python/ipywidgets/ipywidgets/widgets/widget.py

@@ -567,7 +567,7 @@ def set_state(self, sync_data):
         if JUPYTER_WIDGETS_ECHO:
             echo_state = {}
             for attr,value in sync_data.items():
-                if not self.trait_metadata(attr, 'no_echo'):
+                if self.trait_metadata(attr, 'echo_update', default=True):
                     echo_state[attr] = value
             if echo_state:
                 echo_state, echo_buffer_paths, echo_buffers = _remove_buffers(echo_state)

python/ipywidgets/ipywidgets/widgets/widget_upload.py

@@ -133,7 +133,7 @@ class FileUpload(DescriptionWidget, ValueWidget, CoreWidget):
     style = InstanceDict(ButtonStyle).tag(sync=True, **widget_serialization)
     error = Unicode(help='Error message').tag(sync=True)
     value = TypedTuple(Dict(), help='The file upload value').tag(
-        sync=True, no_echo=True, **_value_serialization)
+        sync=True, echo_update=False, **_value_serialization)
 
     @default('description')
     def _default_description(self):