Extract the Coords from Widgets

[chrisbouchard]

Widgets should just be in charge of knowing their size and drawing themselves. We can then create a new type that would wrap a widget and have a position.

Possibly this type would store a Coord internally, but its interface should (also?) allow Positions to be specified for horizontal and vertical alignment.

enum Position {
    FlushStart,
    FlushEnd,
    FromStart(usize),
    FromEnd(usize),
    Middle,
}