Allow users to resize the text window

[mattkimmel]

Right now, the play window (which is an instance of the TextScreen class) is fixed at whatever size is specified when it's initialized. It would be great if players could resize the window while playing, by dragging the corners, or whatever their operating system supports. The current text that's in the window should be preserved.

There's some resizing support in the TextScreen.resizeToFit method. Maybe we could leverage that.