Remove inferred docstring signature and add basic __signature__ support

[maximlt]

Fixes #371 by:

1. removing the automatically inferred docstring signature (e.g. params(x=Number, name=String)) as IPython would no longer leverage it for parameter autocompletion (as noted in the issue) and because it was wrong when the class was having an __init__ method with a signature different than __init__(self, **params)
2. implementing Parameterized.__signature__ as a class property that returns a Signature built from the class Parameters, only when the class (or its superclasses) doesn't implement an __init__ method that is different than the default one __init__(self, **params).

Somewhat surprisingly 2) seemed to have broken only a few tests in Panel and none in HoloViews. The two unit tests that broke in Panel are https://github.com/holoviz/panel/blob/df0b277dca2fd54dd2ae016159adc5c33b61e358/panel/tests/widgets/test_base.py#L25 and https://github.com/holoviz/panel/blob/df0b277dca2fd54dd2ae016159adc5c33b61e358/panel/tests/test_viewable.py#L40, I believe they test that the __signature__ of some Panel objects is actually not modified, as there is some code in Panel that modifies the signature of components. That code in Panel by the way doesn't seem to be tested, given https://github.com/holoviz/panel/blob/df0b277dca2fd54dd2ae016159adc5c33b61e358/panel/tests/conftest.py#L32. Anyway, it seems to me the change done in this PR would not affect Panel, let's double check that however if it's not tested.

I have chosen not to even try to handle custom __init__ in 2) as I'm not even sure it's possible (@jlstevens raised some good points in the issue) and I believe that a better approach for Param in the future would be to add Parameter attributes to control how the Parameter appears in the signature (e.g. positional, hidden) and provide a post init hook so that users don't have to create an __init__ in most cases.

[philippjfr]

Made some minor changes:

• Pull out the default signature into a global variable
• Cache the generated signature on ._param__private

[philippjfr]

Looks good and I'm happy with this conservative approach to auto-generating signatures!