feat: Nested events [wip]

[tlambert03]

this would add support for detected events from nested children in a tree-like evented-dataclass structure. still wip

In [1]: from psygnal._evented_decorator import connect_child_events

In [2]: from dataclasses import dataclass, field

In [3]: from psygnal import evented

In [4]:     @evented
   ...:     @dataclass
   ...:     class Foo:
   ...:         x: int = 1
   ...:
   ...:     @evented
   ...:     @dataclass
   ...:     class Bar:
   ...:         y: int = 2
   ...:         foo: Foo = field(default_factory=Foo)
   ...:

In [5]: bar = Bar()

In [6]: bar.events.connect(print)
Out[6]: <function print at 0x10b631a20>

In [7]: bar.foo.x = 8

In [8]: connect_child_events(bar)

In [9]: bar.foo.x = 12
EmissionInfo(
    signal=<SignalGroup 'FooSignalGroup' with 1 signal on Foo(x=12)>,
    args=(
        EmissionInfo(
            signal=<SignalInstance 'x' on <SignalGroup 'FooSignalGroup' with 1 signal on
Foo(x=12)>>,
            args=(12,),
            attr_name=None
        ),
    ),
    attr_name='foo'
)

[tlambert03]

closing this and picking it up in #298