Add short 'Return' shortcut to view features. add default cols/features for tabbing through, add tabbing for features, add unhighlight short cut.

add funcs to sort feature children by position, get nth and next feature items, print features, get item positions.

add code to sort items to allow tabbing through them sequentially, + code to reset a highlighted features position.

- fixed bug causing error reported in RT ticket #37785
  now uses exon foreach function

only one gdk call now

switch back to single data pointer