When using both an extra control and wxOSX_FILEDIALOG_ALWAYS_SHOW_TYPES
set to 1 to show file types, the controls for the latter stretch towards
the dialog's width.
Fix by not stretching the wxChoice used for filtering and surrounding
the file type controls with stretching spacers instead. This makes
the filter controls appear the same as without an extra control:
horizontally centred and not stretching.
5cc2f6ce9f