Adds a new set of properties to make pushfilesrc output a TIME SEGMENT
(instead of the filesrc BYTE SEGMENT).

When time-segment is set to True the following will happen:
* Seeks are refused (data starts from the beginning of the file)
* The BYTE segment will be replaced by a TIME segment with the values
  specified in the various properties
* The first outgoing buffer will have a timestamp set on it (by default
  it has a value of GST_CLOCK_TIME_NONE)