Added the preferred widget size basework and its GTK+ implementation.
Added a new layout plan for Stack.
Started rewriting Stack to conform to the new layout plan.
Changed Orientation to a bool to prevent invalid values outright.
Wrote the new Stack.setRect().
Fixed some compiler errors in the last few commits.
Added Control.preferredSize() and preferredSize() for all the standard Controls.
Added Stack.preferredSize() and fixed errors in stack.go.
Adjusted main_test.go to make use of SetStretchy(). IT WORKS! :D
Made sure Stack.SetStretchy() (like all other layout-changing functions) cannot be called once the parent Window has been opened.