Rename timeline-object.h to libweston/timeline-object.h

This patch sets up the stage for similarly renaming compositor.h which will
justify this. That patch will be big, so moving timeline-object.h first makes
it easy to see the changes to the build and install directives.

This and all the following moves essentially break the API, so libweston major
is bumped.

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