Atomic updates for multiple GUI properties

[brentyi]

Primary change:

-    prop_name: str
-    prop_value: Any
+    updates: Dict[str, Any]
+    """Mapping from property name to new value."""

Helps avoid some hypothetical errors mentioned in #166.

[jkulhanek]

Looks good to me! We could also consider making the types stronger on the client’s side.

[brentyi]

Ok, I switched it to Partial<GuiAddComponentMessage>. It's not as nice as what you had before, but seems reasonable given the complexity tradeoff of anything stronger.

[jkulhanek]

Thanks!