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/Applications/KeyboardMapper/KeyboardMapperWidget.cpp

@@ -28,7 +28,7 @@ void KeyboardMapperWidget::create_frame()
 {
     set_fill_with_background_color(true);
     set_layout<GUI::VerticalBoxLayout>();
-    layout()->set_margins({ 4, 4, 4, 4 });
+    layout()->set_margins(4);
 
     auto& main_widget = add<GUI::Widget>();
     main_widget.set_relative_rect(0, 0, 200, 200);