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/MultiView.cpp

@@ -20,7 +20,7 @@ namespace GUI {
 MultiView::MultiView()
 {
     set_active_widget(nullptr);
-    set_content_margins({ 2, 2, 2, 2 });
+    set_content_margins(2);
     m_icon_view = add<IconView>();
     m_table_view = add<TableView>();
     m_columns_view = add<ColumnsView>();