simplify display of headerline and setting of active band

- rename bandindex_normal and bandindex_warc to bi_normal/warc
- show bands in header line according to the list of band indices
  in bi_normal or bi_warc
- simplify highlighting the  actual band
  (do it now on the fly while printing the line in first place.)
factor out common code into 'display_header()'

- gets list of bands to show as parameter