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/Terminal/TerminalSettingsWindow.gml

@@ -2,7 +2,7 @@
     fill_with_background_color: true
 
     layout: @GUI::VerticalBoxLayout {
-        margins: [4, 4, 4, 4]
+        margins: [4]
     }
 
     @GUI::GroupBox {
@@ -10,7 +10,7 @@
         shrink_to_fit: true
 
         layout: @GUI::VerticalBoxLayout {
-            margins: [16, 6, 6, 6]
+            margins: [16, 6, 6]
         }
 
         @GUI::RadioButton {
@@ -34,7 +34,7 @@
         shrink_to_fit: true
 
         layout: @GUI::VerticalBoxLayout {
-            margins: [16, 6, 6, 6]
+            margins: [16, 6, 6]
         }
 
         @GUI::OpacitySlider {
@@ -50,7 +50,7 @@
         shrink_to_fit: true
 
         layout: @GUI::VerticalBoxLayout {
-            margins: [16, 6, 6, 6]
+            margins: [16, 6, 6]
         }
 
         @GUI::SpinBox {
@@ -66,7 +66,7 @@
         shrink_to_fit: true
 
         layout: @GUI::VerticalBoxLayout {
-            margins: [16, 6, 6, 6]
+            margins: [16, 6, 6]
         }
 
         @GUI::ComboBox {