GtkFileChooser: Don't fill location_entry if we just populated the file list

GtkFileChooser: Don't fill location_entry if we just populated the file list

Only fill the location entry with the file name of the tree view's
selected file when the selection was done by the user.

When the file chooser's action is GTK_FILE_CHOOSER_ACTION_OPEN, it
selects the first file in the tree view once loading has finished. For
this case we don't want it to insert the file name in the location
entry, as it hinders efficient navigation using the location entry. To
achieve this, use a priv flag to keep track of whether the
selection-changed signal was caused by the file chooser itself.

https://bugzilla.gnome.org/show_bug.cgi?id=386569