Refactor constructing FrameTime

[mgoral]

It's a little messy right now, better approach would be something like this:

class FrameTime:
  def __init__(self, fps): pass # empty (0) FT

  @classmethod
  def FromFrame(cls, frame, fps): pass # from frame format

  @classmethod
  def FromSeconds(cls, secs, fps): pass # from seconds

  @classmethod
  def FromStrptime(cls, s, fps, fmt): pass # something like parsing "1:01:02", but should accept strings with subset of strptime format