Userland+LibGUI: Add shorthand versions of the Margins constructor

This allows for typing [8] instead of [8, 8, 8, 8] to specify the same
margin on all edges, for example. The constructors follow CSS' style of
specifying margins. The added constructors are:

- Margins(int all): Sets the same margin on all edges.
- Margins(int vertical, int horizontal): Sets the first argument to top
  and bottom margins, and the second argument to left and right margins.
- Margins(int top, int vertical, int bottom): Sets the first argument to
  the top margin, the second argument to the left and right margins,
  and the third argument to the bottom margin.

Userland/Libraries/LibGUI/MessageBox.cpp

@@ -89,7 +89,7 @@ void MessageBox::build()
     widget.set_layout<VerticalBoxLayout>();
     widget.set_fill_with_background_color(true);
 
-    widget.layout()->set_margins({ 8, 8, 8, 8 });
+    widget.layout()->set_margins(8);
     widget.layout()->set_spacing(6);
 
     auto& message_container = widget.add<Widget>();