gtkplacessidebar: Don't change location if clicked on eject button

Currently we change the current location if we click the eject button of
a mount.

Check whether the user actually clicked the eject button and don't
change location in that case.

https://bugzilla.gnome.org/show_bug.cgi?id=737983
gtkplacessidebar: Open $HOME after unmounting or ejecting

When a drive is ejected or a volume unmounted the current directory
doesn't change most of the times being empty or being a directory that
user shouldn't take care about, like /run/media

Seems more useful to change to $HOME directory in that case so the user
can see something useful and familiar just after unmounting.

https://bugzilla.gnome.org/show_bug.cgi?id=737983