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/TextEditor/TextEditorWindow.gml

@@ -36,7 +36,7 @@
 
         layout: @GUI::VerticalBoxLayout {
             spacing: 2
-            margins: [5, 5, 5, 5]
+            margins: [5]
         }
 
         @GUI::Widget {