file-conflict-dialog: Prevent replacing a directory with a symbolic link

If the user tries to replace a folder with a symbolic link with the same
name,the content of the destination folder was being deleted.

To fix this behaviour, the UI disables the replace button allowing the
user only to skip or to rename in order to avoid the deletion of the
destination folder's content.

https://bugzilla.gnome.org/show_bug.cgi?id=632323