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).
It does almost nothing (just puts a non-function button on the screen),
but it will help develop the IMUI code and, of course, come to help with
debugging in general.
