bookmarks-window: always get the bookmark name from the name field.

Always get the current value in the name field when editing a bookmark.
This will prevent passing NULL to nautilus_bookmark_new when the user
edits only the URI field. I suspect that passing NULL to
nautilus_bookmark_new causes nautilus to create a new name from URI,
thus overwriting the current bookmark name.

https://bugzilla.gnome.org/show_bug.cgi?id=576976