Rename wayland-compositor to weston

This rename addresses a few problems around the split between core
Wayland and the wayland-demos repository.

1) Initially, we had one big repository with protocol code, sample
compositor and sample clients.  We split that repository to make it
possible to implement the protocol without pulling in the sample/demo
code.  At this point, the compositor is more than just a "demo" and
wayland-demos doesn't send the right message.  The sample compositor
is a useful, self-contained project in it's own right, and we want to
move away from the "demos" label.

2) Another problem is that the wayland-demos compositor is often
called "the wayland compsitor", but it's really just one possible
compositor.  Existing X11 compositors are expected to add Wayland
support and then gradually phase out/modularize the X11 support, for
example.  Conversely, it's hard to talk about the wayland-demos
compositor specifically as opposed to, eg, the wayland protocol or a
wayland compositor in general.

We are also renaming the repo to weston, and the compositor
subdirectory to src/, to emphasize that the main "output" is the
compositor.