Allow StBoxLayout to shrink down to its minimum size

When a StBoxLayout is allocated a size less than its natural size,
think "shrink" needs to be divided among the children that have
a smaller minimum size than natural size.

This is done by preferentially shrinking the children that are most
expanded from their minimum size and then increasing that set of
children until we've found enough total shrink.

A new method is used of allocating children at integral sizes - instead
of rounding the per-child extra amount to an integer (which causes
cumulative round-off errors), compute the position as we go along in
floats and round individually for each child widget.

Extend the box-layout test to include of a test of a box being set
to various widths, starting quite narrow.

http://bugzilla.moblin.org/show_bug.cgi?id=6311
https://bugzilla.gnome.org/show_bug.cgi?id=595995