placesview: vertically align path labels

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