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. :^)

Applications/FileManager/FileManagerWindow.gml

@@ -19,8 +19,7 @@
 
             @GUI::TextBox {
                 name: "location_textbox"
-                vertical_size_policy: "Fixed"
-                preferred_height: 22
+                fixed_height: 22
             }
         }
         @GUI::ToolBar {
@@ -37,8 +36,7 @@
 
         @GUI::TreeView {
             name: "tree_view"
-            horizontal_size_policy: "Fixed"
-            preferred_width: 175
+            fixed_width: 175
         }
 
     }