bgo#486839 - Fix path bar size group in the file chooser

bgo#486839 - Fix path bar size group in the file chooser

Since the location button is hidden in save mode, we need to add the
path bar to the size group too. The location button still has to be in
the group though, because it's larger than the path bar (when
shown). Instead of using the recent/search icons, add their hboxes so
that themed widget spacings don't introduce variations.