Enhance layout of prefs-dialog.

data/panel-osd-settings.ui

@@ -16,6 +16,10 @@
     <property name="height_request">1</property>
     <property name="can_focus">False</property>
     <property name="valign">start</property>
+    <property name="margin_left">10</property>
+    <property name="margin_right">10</property>
+    <property name="margin_top">10</property>
+    <property name="margin_bottom">10</property>
     <property name="vexpand">True</property>
     <property name="orientation">vertical</property>
     <child>