By default, horizontal and vertical layouts expand to fill their parent
in their on-axis direction (horizontally for horizontal layouts), but
fit to their child views in their off-axis.
Flexible space views take advantage of auto-expansion, pushing sibling
views such that the grandparent view is filled on the parent view's
on-axis, and the parent view is filled by the space in the parent view's
off-axis. Flexible views currently have a background fill, allowing them
to provide background filling of the overall view with minimal overdraw
(ancestor views don't need to have any fill at all).
TextContent seems redundant at this stage since a text view is always
sized to its content, and PercentOfParent doesn't work yet. Pixels
definitely works and Null seems to work in that it does no sizing or
positioning. Vertical layout is supported but not yet tested, similar
for ChildrenSum, but I can have two buttons side by side.
