SidebarRow: Add Unmount tooltip to eject_button

It was reported that the lack of a tooltip made its purpose unclear.
This can be solved by just copying PlacesViewRow’s eject_button tooltip.

https://bugzilla.gnome.org/show_bug.cgi?id=766909

gtk/ui/gtksidebarrow.ui

@@ -46,6 +46,7 @@
                     <property name="valign">center</property>
                     <property name="margin-start">4px</property>
                     <property name="no-show-all">1</property>
+                    <property name="tooltip-text" translatable="yes">Unmount</property>
                     <child>
                       <object class="GtkImage">
                         <property name="visible">1</property>