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/Frame.cpp

@@ -42,7 +42,7 @@ void Frame::set_frame_thickness(int thickness)
     if (m_thickness == thickness)
         return;
     m_thickness = thickness;
-    set_content_margins({ thickness, thickness, thickness, thickness });
+    set_content_margins(thickness);
 }
 
 void Frame::paint_event(PaintEvent& event)