placesview: show Disconnect instead of Eject

Since we're dealing with networks, terms like "Eject" or
the eject button are misleading, since we're not actually
ejecting but disconnecting.

Fix that by showing the appropriate icon and tooltip.

gtk/ui/gtkplacesviewrow.ui

@@ -72,7 +72,7 @@
                 <property name="valign">center</property>
                 <property name="tooltip-text" translatable="yes">Unmount</property>
                 <child>
-                  <object class="GtkImage">
+                  <object class="GtkImage" id="eject_icon">
                     <property name="visible">True</property>
                     <property name="icon_name">media-eject-symbolic</property>
                     <property name="icon_size">1</property>