VUI: More layout boxes for aligning widgets

[kpreid]

In order to to have good and easy-to-define UI layout, we need more layout functionality than the current basis of “minimum size” boxes only.

In particular, I think that we need something I don't yet have a good name for but I've been calling “base-box”, which is a “preferred” size that is neither minimal nor maximal, which primarily flows from parents to children during layout. For example,

• The base-box of the HUD layout should be a box that fits fully within the area of the screen, separate from margin/border/overscan space, and in the depth axis, defines the plane in which the HUD widgets should lie.
• In a vertical stack of buttons, the base-box is chosen to fit the widest button label, and makes all the buttons that wide.

Widgets can stick out past the base-box as necessary to fit their content. LayoutGrants go from having only a maximum bounds they are allowed to draw in, to also having the base-box which they can try to fit themselves into.