libweston: Define head, output and compositor group

Also, add tag symbols related to 'weston_head'.

The bridge between sphinx and doxygen (breathe) has a useful directive:
doxygengroup. By using it we can scoop out symbols we'd like to display
documentation from/of.

At the same time some bits of the code has been using '\memberof' (a
doxygen command useful in C code to establish class like
relationship between objects and functions) but this seems not to be
recognized by the sphinx bridge.

Until we find a better solution, we replace '\memberof' command with
'\ingroup' one as to tag the symbols with an "object". This patch does
that for 'weston_head' object.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

include/libweston/libweston.h

@@ -173,6 +173,8 @@ enum dpms_enum {
  *
  * This object represents a monitor (hardware backends like DRM) or a window
  * (windowed nested backends).
+ *
+ * \ingroup head
  */
 struct weston_head {
 	struct weston_compositor *compositor;	/**< owning compositor */