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/InterruptsWidget.cpp

@@ -15,7 +15,7 @@ InterruptsWidget::InterruptsWidget()
 {
     on_first_show = [this](auto&) {
         set_layout<GUI::VerticalBoxLayout>();
-        layout()->set_margins({ 4, 4, 4, 4 });
+        layout()->set_margins(4);
 
         Vector<GUI::JsonArrayModel::FieldSpec> interrupts_field;
         interrupts_field.empend("interrupt_line", "Line", Gfx::TextAlignment::CenterRight);