Improve caps setters API

Improve caps setters API

This patch adds gst_caps_set_value() and allows gst_caps_set_simple() to
work on non-simple caps. See the API documentation for the functions
about what they do.

The intention of these changes is to ease working with caps in caps
transform functions. An example for this would be ffmpegcolorspace,
where the caps transform function could be changed to look roughly like
this (pseudocode ahead):
  result = gst_caps_copy (template_caps);
  value = gst_structure_get_value (gst_caps_get_structure (caps, 0),
    "widh");
  gst_caps_set_value (result, value);
  /* same for height, framerate and par */
  return caps;
which is much cleaner and easier to understand than the current code.

https://bugzilla.gnome.org/show_bug.cgi?id=597690