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>