Add container prop to AppBar

[WiXSL]

Implements #6174

[djhi]

As we want to keep the default behavior, I think having to pass false explicitly is not very intuitive. What do you think about adding a stickOnScroll prop which would be false by default? Maybe there is a better name for it though ;)

[fzaninotto]

Why don't you call this prop container and let users pass a Fragment instead of HideOnScroll?

[djhi]

😂 🎉

[fzaninotto]

thanks!