This patch removes size policies and preferred sizes, and replaces them
with min-size and max-size for each widget.
Box layout now works in 3 passes:
1) Set all items (widgets/spacers) to their min-size
2) Distribute remaining space evenly, respecting max-size
3) Place widgets one after the other, adding spacing in between
I've also added convenience helpers for setting a fixed size (which is
the same as setting min-size and max-size to the same value.)
This significantly reduces the verbosity of widget layout and makes GML
a bit more pleasant to write, too. :^)
