Add support for Annotated type

Pulling out this great tip by @VolkerH from https://github.com/napari/magicgui/issues/20#issuecomment-647888706 into a new issue. We can use the Annotated type (PEP 593) available through typing_extensions for parameter-specific metadata.

so, as demonstrated by @jni in https://github.com/napari/magicgui/issues/20#issuecomment-648504543, we could put argument-specific options directly in the type hint:

@magicgui(arg={'minimum': 10, 'maximum': 100})
def my_func(arg=50): ...

# could become this:
@magicgui
def my_func(arg: Annotated[int, {'minimum': 10, 'maximum': 100}] = 50): ...

[tlambert03]

implemented in #43 …@magicgui(arg={'min': 10, 'max': 100}) will continue to be the “promoted” API… but arg: Annotated[int, {'min': 10, 'max': 100}] = 50 will also work. moreover, the magicgui.signature.MagicParameter is a subclass of inspect.Parameter that bundles the widget options in an Annotated type in the Parameter.annotation

[tlambert03]

this should be the fallback way to do this, while kwargs to magicgui should be the recommended way. … That’s the distinction I’m trying to make and one which I would hope magicgui would recommend…

understood and totally agree. “magicgui” will recommend that 😃

I think I’m probably focusing (in my head) a bit more at the moment on the usefulness of Annotated in an inspect.Signature as the internal representation that magicgui would use to pass around this info, in which case “supporting” it here would be trivial. but I definitely agree, I wouldn’t want to start changing all the examples to encourage people to use Annotated for GUI-specific things in their function signatures