ensure LayerSelect updates

* overly greedy, but ensures the layer items in the legend are updated for a new subset.

jdaviz/core/template_mixin.py

@@ -1497,8 +1497,8 @@ def __init__(self, plugin, items, selected, viewer,
         self.hub.subscribe(self, SubsetCreateMessage,
                            handler=lambda _: self._on_subset_created())
         # will need SubsetUpdateMessage for name only (style shouldn't force a full refresh)
-        # self.hub.subscribe(self, SubsetUpdateMessage,
-        #                    handler=lambda _: self._update_layer_items())
+        self.hub.subscribe(self, SubsetUpdateMessage,
+                           handler=lambda _: self._update_layer_items())
         self.hub.subscribe(self, SubsetDeleteMessage,
                            handler=lambda _: self._update_layer_items())
 
@@ -1714,6 +1714,8 @@ def _on_data_added(self, msg=None):
     @observe('filters', 'sort_by')
     def _update_layer_items(self, msg={}):
         # NOTE: _on_layers_changed is passed without a msg object during init
+        # TODO: if the message is a SubsetUpdateMessage, only act on those that require
+        # an update
         # TODO: Handle changes to just one item without recompiling the whole thing
         manual_items = [{'label': label} for label in self.manual_options]
         # use getattr so the super() call above doesn't try to access the attr before