Improvement to watch function registration for methods

[okomestudio]

We want watch function registration to be more flexible. In particular, we want to make this possible with class, so that different actions can be associated with the object state:

class DB:
    @conf.watch("db", actions=["ADDED"])
    def setup(self, action, old, new):
        ...

    @conf.watch("db", actions=["MODIFIED"])
    def update(self, action, old, new):
        ...

    @conf.watch("db", actions=["REMOVED"])
    def teardown(self, action, old, new):
        ...