LibGUI: Rewrite layout system in terms of min and max sizes

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. :^)

Libraries/LibGUI/Label.cpp

@@ -119,8 +119,7 @@ void Label::paint_event(PaintEvent& event)
 
 void Label::size_to_fit()
 {
-    set_size_policy(SizePolicy::Fixed, SizePolicy::Fill);
-    set_preferred_size(font().width(m_text), 0);
+    set_fixed_width(font().width(m_text));
 }
 
 }