Rename compositor.h to libweston/libweston.h

The main idea is to make libweston users use the form

 #include <libweston/libweston.h>

instead of the plain

 #include <compositor.h>

which is prone to name conflicts. This is reflected both in the installed
files, and the internal header search paths so that Weston would use the exact
same form as an external project using libweston would.

The public headers are moved under a new top-level directory include/ to make
them clearly stand out as special (public API).

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>