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/DisplaySettings/BackgroundSettings.gml

@@ -2,7 +2,7 @@
     fill_with_background_color: true
 
     layout: @GUI::VerticalBoxLayout {
-        margins: [8, 8, 8, 8]
+        margins: [8]
     }
 
     @DisplaySettings::MonitorWidget {

Userland/Applications/DisplaySettings/DesktopSettings.gml

@@ -2,12 +2,12 @@
     fill_with_background_color: true
 
     layout: @GUI::VerticalBoxLayout {
-        margins: [8, 8, 8, 8]
+        margins: [8]
     }
 
     @GUI::GroupBox {
         layout: @GUI::VerticalBoxLayout {
-            margins: [24, 16, 6, 16]
+            margins: [24, 16, 6]
         }
 
         title: "Workspaces"
@@ -17,7 +17,7 @@
             fixed_height: 32
 
             layout: @GUI::HorizontalBoxLayout {
-                margins: [6, 6, 6, 6]
+                margins: [6]
             }
 
             @GUI::Label {
@@ -57,7 +57,7 @@
             }
             @GUI::Widget {
                 layout: @GUI::VerticalBoxLayout {
-                    margins: [6, 6, 6, 6]
+                    margins: [6]
                 }
                 @GUI::Label {
                     text: "Use the Ctrl+Alt+Arrow hotkeys to move between workspaces."

Userland/Applications/DisplaySettings/FontSettings.gml

@@ -2,7 +2,7 @@
     fill_with_background_color: true
 
     layout: @GUI::VerticalBoxLayout {
-        margins: [8, 8, 8, 8]
+        margins: [8]
         spacing: 8
     }
 

Userland/Applications/DisplaySettings/MonitorSettings.gml

@@ -2,7 +2,7 @@
     fill_with_background_color: true
 
     layout: @GUI::VerticalBoxLayout {
-        margins: [8, 8, 8, 8]
+        margins: [8]
     }
 
     @DisplaySettings::MonitorWidget {
@@ -34,7 +34,7 @@
 
     @GUI::GroupBox {
         layout: @GUI::VerticalBoxLayout {
-            margins: [24, 16, 6, 16]
+            margins: [24, 16, 6]
         }
 
         title: "Screen settings"

Userland/Applications/DisplaySettings/main.cpp

@@ -45,7 +45,7 @@ int main(int argc, char** argv)
     auto& main_widget = window->set_main_widget<GUI::Widget>();
     main_widget.set_fill_with_background_color(true);
     main_widget.set_layout<GUI::VerticalBoxLayout>();
-    main_widget.layout()->set_margins({ 4, 4, 4, 4 });
+    main_widget.layout()->set_margins(4);
     main_widget.layout()->set_spacing(6);
 
     auto& tab_widget = main_widget.add<GUI::TabWidget>();