ClockSettings: Add a preview of the current time format

When writing a custom format especially, it's nice to be able to see how
it will look as you write it.

Userland/Applications/ClockSettings/ClockSettingsWidget.gml

@@ -8,7 +8,7 @@
     @GUI::GroupBox {
         title: "Time Format"
         shrink_to_fit: false
-        fixed_height: 200
+        fixed_height: 240
         layout: @GUI::VerticalBoxLayout {
             margins: [16, 8, 8]
         }
@@ -50,5 +50,25 @@
                 name: "custom_format_input"
             }
         }
+
+        @GUI::Widget {
+            layout: @GUI::HorizontalBoxLayout {
+                spacing: 4
+            }
+
+            @GUI::Label {
+                text: "Preview:"
+                text_alignment: "CenterLeft"
+            }
+
+            @GUI::Frame {
+                layout: @GUI::VerticalBoxLayout {}
+
+                @GUI::Label {
+                    name: "clock_preview"
+                    text: "12:34:56"
+                }
+            }
+        }
     }
 }