A "renderer" parameter for transforming the underlying value into a nicer display value

[Etherian]

On elements where it makes sense, add a parameter that takes a callback which transforms the widget's underlying values into displayed ones that are nicer-looking and easier for the user to understand.

For example, I have a ComboBox of dictionaries like

from magicgui import ComboBox

ComboBox(
    value={'color': 'red', 'shape': 'square'},
    choices=[
        {'color': 'red', 'shape': 'square'},
        {'color': 'yellow', 'shape': 'circle'}
    ],
)

It would be nice if I could display something nicer for the user with

ComboBox(
    value={'color': 'red', 'shape': 'square'},
    choices=[
        {'color': 'red', 'shape': 'square'},
        {'color': 'yellow', 'shape': 'circle'}
    ],
    renderer=lambda val: f'val["color"] val["shape"]'
)

[tlambert03]

yeah, I like that a lot! feel free to make a PR, or I will try to get around to it in the next few weeks.