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/SystemMonitor/ProcessMemoryMapWidget.cpp

@@ -47,7 +47,7 @@ public:
 ProcessMemoryMapWidget::ProcessMemoryMapWidget()
 {
     set_layout<GUI::VerticalBoxLayout>();
-    layout()->set_margins({ 4, 4, 4, 4 });
+    layout()->set_margins(4);
     m_table_view = add<GUI::TableView>();
     Vector<GUI::JsonArrayModel::FieldSpec> pid_vm_fields;
     pid_vm_fields.empend(