Provide more direct "autowidget generation" without requiring function body.

[tlambert03]

in using magicgui to create a single widget in napari/napari#1069, even though they didn't a function with logic in the body, there was no choice but to do:

@magicgui(label={'choices': labels})
def label_selection(label):
    return label

while the original conception of magicgui was to provide a GUI for simple functions, this makes me think that we should also offer one-off widgets that still abstract away the widget backend (one shouldn't need to know any Qt methods if they don't want to), while having a bit more direct access to Type->Widget lookups. something like:

label_selection = magicgui.make_widget(choices=labels)

[tlambert03]

Possible now. #43 introduced both magicgui.widgets.create_widget as well as the direct widget creation API