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

@@ -37,6 +37,7 @@
                 <property name="valign">center</property>
                 <property name="margin-start">4px</property>
                 <property name="icon-name">media-eject-symbolic</property>
+                <property name="tooltip-text" translatable="yes">Unmount</property>
                 <style>
                   <class name="sidebar-button"/>
                 </style>