Make sure all trait listeners get unhooked

There are some cases where we set up trait listeners but never ultimately unhook them (i.e. we never call `.observe(..., remove=True))`. In many of these cases the use of the code is transient (e.g. examples / tests), but in any case it is best to always be safe. Any code which isn't transient this is very important as it could lead to strange bugs.

Search the repository for trait listener setup and calls to .observe(..., remove=True), including examples and tests, then identify listeners that can outlive their intended use. Done means every listener established by the affected code has a corresponding unhook path, including transient usage where practical.

Written by the indexing model from the issue text.