x264enc: Refactor code in preparation for presets/tunings

x264enc: Refactor code in preparation for presets/tunings

- Make defaults append the appropriate default value to a string. This is
  needed to differentiate between something user-set and the actual prop
  default.
- Add an internal option string to which _set_property () cases append for the
  majority of properties.
- Use gst_x264_enc_parse_options () to clean up application of settings. This
  will make order of application with respect to the presets and tunings quite
  simple.

Addresses part of bug #607798