Adwaita: theme shortcuts windows a bit

Add some space around the content.

gtk/theme/Adwaita/_common.scss

@@ -4372,6 +4372,10 @@ cursor-handle {
 
 .context-menu { font: initial; } // Decouple the font of context menus from their entry/textview
 
+shortcuts-section {
+  margin: 20px;
+}
+
 // shortcut window keys
 .keycap {
   min-width: 20px;