Vertical Offset, Shadows on all modes & new Overlay Mode

[luismarquesfreire]

These 3 features were all missing to match the project I was working on. I needed to have a vertical offset, without scale, rounded corners and shadows and then, overlaying the revealing content. This changes solves this.
Added a new example with a different background to better show the concept.

[Dn-a]

@luismarquesfreire your idea is interesting.
I have seen that the offset works only from top to bottom.
Why not also add the possibility to set it from bottom to top?

PS: before making a push, run flutter format .

[Dn-a]

@luismarquesfreire hi!
If at the moment you have no way to implement this feature and you give me the consent, i can implement the vertical offset in the new release. thanks

[luismarquesfreire]

@Dn-a yes please, go ahead, i'm really busy at the moment, hope my contribution helps.

[Dn-a]

@luismarquesfreire New release 0.5.4 with new features. check and leave feedback.

[luismarquesfreire]

Great @Dn-a I will check that thanks.