Enable custom icon elements in drawer items, make drawer header fixed

[sraka1]

Drawer/Section change is pretty straightforward, since icon is fed into a ListItem as the leftElement, and ListItem leftElement supports a custom element, the icon should too, I guess.

Drawer/Drawer - looking at the Gmail app, the Header section is fixed (does not scroll) - this contradicts https://material.io/guidelines/patterns/navigation-drawer.html#navigation-drawer-content, where it says "The navigation drawer scrolls in the same way a view scrolls.". However, I think the Gmail app behaviour is more intuitive. The change is a bit trivial, as it always assumes the first item would be the header, which is not a must, but should suffice in most cases...

[xotahal]

@sraka1 Hey, thank you for support .. anyway, could we do it with prop? Sth like staticHeader: true ... which will be set to false by default ...