[POC] Material theme extensions

[bdlukaa]

#93

The class MixProperty<T> was created, which will be in every mix attribute. With it, it's possible to use values that depends on the current BuildContext. This means it's possible to do both bgColor(Colors.white) and bgColor(theme<MyColors, Color>((colors) => colors.white)).

Added theme, a function that fetches ThemeExtensions from the closest Theme widget. Also a basic example was added to the /demo app. To try it, change the app theme mode (light/dark) and see the "Gradient Box" color change as the theme changes.

Another advantage of this approach is that we now can make use of Material's AnimatedTheme. When a property in your ThemeExtension changes, its lerp function is called and is animated accordingly to the theme. (animate() works with it too, in case you want to delay the animation)

For now, only the following utils are affected: bgColor, margin and height

[bdlukaa]

Even if we don't want to make use of ThemeExtensions in mix directly, I believe adding a way to access the BuildContext is a must.

However, with the implementation above, we lose the dart type-safety (by using dynamic).

@leoafarias I'd love to hear your thoughts on this

[leoafarias]

Thanks for the work on this @bdlukaa. We have already 2 abstractions for values of context. Dto and Tokens (Which might rename Refs, want to see how these fit together.

Also the example allows to pass properties directly to a Box, but since Box is a primitive, I don't think a very similar properties is likely on the primitives themselves. For example on the headless widgets this might be different.

I believe this extensions should work similar to the tokens, or see if we can abstract the tokens to take advantage of ThemeExtensions instead of a separate values on MixTheme.