[Done 2.8 Beta] Lua: Track.has_Group_Parent

Add has_group_parent in track objects. Otherwise you need to make tedious checks to avoid error in many situations?

Or better, return nil instead of:

*** [string “oprint(renoise.song().tracks[3].group_paren…”]:1: unknown property or function ‘group_parent’ for an object of type ‘Track’
*** stack traceback:
*** [C]: in function ‘_error’
*** [string “do…”]:47: in function <[string “do…”]:35>
*** [string “oprint(renoise.song().tracks[3].group_paren…”]:1: in main chunk