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/FilePickerDialog.gml

@@ -2,14 +2,14 @@
     fill_with_background_color: true
 
     layout: @GUI::HorizontalBoxLayout {
-        margins: [4, 4, 4, 4]
+        margins: [4]
         spacing: 3
     }
 
     @GUI::Widget {
         shrink_to_fit: true
         layout: @GUI::VerticalBoxLayout {
-            margins: [0, 4, 0, 4]
+            margins: [0, 4]
         }
 
         @GUI::Label {
@@ -24,7 +24,7 @@
             fill_with_background_color: true
 
             layout: @GUI::VerticalBoxLayout {
-                margins: [2, 2, 2, 2]
+                margins: [2]
                 spacing: 0
             }
         }