media-player: Remove margin

This makes it the same width as notifications instead of slightly
smaller which looked very odd.
Signed-off-by: Guido Günther <guido.gunther@puri.sm>

src/ui/media-player.ui

@@ -4,10 +4,8 @@
   <requires lib="gtk+" version="3.20"/>
   <template class="PhoshMediaPlayer" parent="GtkGrid">
     <property name="can_focus">False</property>
-    <property name="margin_left">5</property>
-    <property name="margin_right">5</property>
-    <property name="margin_top">5</property>
-    <property name="margin_bottom">5</property>
+    <property name="margin-top">5</property>
+    <property name="margin-bottom">5</property>
     <property name="column_homogeneous">True</property>
     <child>
       <object class="GtkBox">