Support entering/exiting alternate screen

[JakeWharton]

No description provided.

[JakeWharton]

To start with I think an entry-point boolean is enough. Things like the snake game can make use of it.

Still can't really think of a good design for dynamically switching in and out of the mode, though. It has all kinds of other implications on things like static logging, what happens to nodes that are "outside" the alternate screen, how to handle multiple and/or nesting, etc.