Actor: Add content methods.

* clutter/src/actor.[hg|ccg]: Add get/set_content(),
  get/set_content_gravity(), get/set_content_scaling_filters(),
  and get/set_content_repeat() and some relevant enums.
* clutter/src/types.hg: Add an enum here.
* codegen/m4/convert_clutter.m4: Mention the enums here.