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/PixelPaint/CreateNewGuideDialog.gml

@@ -6,7 +6,7 @@
 
     @GUI::Widget {
         layout: @GUI::VerticalBoxLayout {
-            margins: [4,4,4,4]
+            margins: [4]
         }
 
         @GUI::GroupBox {
@@ -14,7 +14,7 @@
             shrink_to_fit: true
             
             layout: @GUI::HorizontalBoxLayout {
-                margins: [20,10,10,10]
+                margins: [20, 10, 10]
             }
 
             @GUI::RadioButton {
@@ -31,7 +31,7 @@
 
     @GUI::Widget {
         layout: @GUI::HorizontalBoxLayout {
-            margins: [4,4,4,4]
+            margins: [4]
         }
         shrink_to_fit: true
 
@@ -50,7 +50,7 @@
         max_height: 24
 
         layout: @GUI::HorizontalBoxLayout {
-            margins: [4,4,4,4]
+            margins: [4]
         }
 
         @GUI::Widget {