Add ability to replace `style` attribute

[hyuri]

What is the problem or limitation you are having?

I want to be able to change a widget's style, by passing another variable containing Pack() styling.

Right now, the style is not changed if I do, for example:

my_style_one = Pack(...)
my_style_two = Pack(...)

label = toga.Label("Step 1", style=my_style_one)

label.style = my_style_two

Even if I use label.refresh() afterwards.

Describe the solution you'd like

Be able to re-assign a style like so:

label.style = new_style

Describe alternatives you've considered

A method for updating properties, such as:

label.update("style", new_style)

Additional context

No response

[freakboy3742]

widget.style.update(**new style) already exists.

However, I agree that assignment of a complete style should also be possible.

[drewlar]

Going to picking this up at the Pycon US sprint!