convert dartpad over to use styled_widget

[devoncarew]

• convert dartpad over to use styled_widget

This converts uses of Padding and Center to use extension method from package:styled_widget. It may be interesting convert more code over to use the styled_widget extension methods, to better gauge changes to the readability of the code, ...

Note that it looks like some parts of the github diff get confused by the changes - the delta looks bigger than it is.

I generally like the changes; it reduces the level of nesting, making it easier to work within 80 cols. And suspect that I'd like having the modifier methods trail after the main widgets, esp. if we used more of the extension methods (so, Foo(...).padding(...).center().background(...)).

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