Now you don't need to pass the navigator around everywhere.

Fixes #1669

MaterialApp assumes that you're using material design.

Also move radial reaction and fix imports for stats box.

These are now part of material.dart.

Added horizontal and vertical alignment properties to Stack so that the origin of non-positioned children can be specified. Currently all of the non-positioned children just end up with their top-left at 0,0. Now, for example, you can center the children by specifying verticalAlignment: 0.5, horizontalAlignment: 0.5.

Added IndexedStack which only paints the stack child specified by the index property. Since it's a Stack, it's as big as the biggest non-positioned child. This component will be essential for building mobile drop down menus.

Added a (likely temporary) example that demonstrates IndexedStack.