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/ResizeCorner.cpp

@@ -79,8 +79,7 @@ ResizeCorner::ResizeCorner()
 {
     set_override_cursor(Gfx::StandardCursor::ResizeDiagonalTLBR);
     set_background_role(ColorRole::Button);
-    set_size_policy(SizePolicy::Fixed, SizePolicy::Fixed);
-    set_preferred_size(16, 18);
+    set_fixed_size(16, 18);
 }
 
 ResizeCorner::~ResizeCorner()