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. :^)

Services/NotificationServer/NotificationWindow.cpp

@@ -105,8 +105,7 @@ NotificationWindow::NotificationWindow(const String& text, const String& title,
     text_label.set_text_alignment(Gfx::TextAlignment::CenterLeft);
 
     auto& right_container = widget.add<GUI::Widget>();
-    right_container.set_size_policy(GUI::SizePolicy::Fixed, GUI::SizePolicy::Fill);
-    right_container.set_preferred_size(36, 0);
+    right_container.set_fixed_width(36);
     right_container.set_layout<GUI::HorizontalBoxLayout>();
 
     on_close_request = [this] {