placesview: vertically align path labels

The current situation is somewhat sad, with the path
label totally misaligned throughout the rows.

This is fixed by using a size group for the path labels,
so they all have the same allocated size (with the max
of 15 chars). Also, instead of hiding the eject button,
set it child-invisible, so it is hidden and yet it's size
is allocated by GtkBox.

https://bugzilla.gnome.org/show_bug.cgi?id=757303

gtk/gtkplacesviewrowprivate.h

@@ -24,6 +24,7 @@
 #endif
 
 #include "gtkwidget.h"
+#include "gtksizegroup.h"
 #include "gtklistbox.h"
 
 G_BEGIN_DECLS
@@ -53,6 +54,9 @@ gboolean           gtk_places_view_row_get_is_network            (GtkPlacesViewR
 void               gtk_places_view_row_set_is_network            (GtkPlacesViewRow   *row,
                                                                   gboolean            is_network);
 
+void               gtk_places_view_row_set_path_size_group       (GtkPlacesViewRow   *row,
+                                                                  GtkSizeGroup       *group);
+
 G_END_DECLS
 
 #endif /* GTK_PLACES_VIEW_ROW_H */