Reorganize libweston source files

The libweston file structure needs clarifying and namespacing. Public headers should be clearly separate from the rest. Public headers should be included with namespaced names. Backend files should be named as such. Some backends and GL-renderer could use splitting into multiple files, hence they need their own sub-directories.

Changing all that will obviously cause churn both internally and to libweston users. The intention is to reorganize everything quickly and hope it will last for the next decade.

See also: #219 (closed)

• Move trivial public headers: timeline-object.h, windowed-output-api.h, plugin-registry.h.
• Rename compositor.h to libweston/libweston.h.
• Rename compositor-*.h to libweston/backend-*.h
• Move libwayland-desktop.h, but should it go to libweston/include/libweston/libweston-desktop.h or libweston-desktop/include/libweston/libweston-desktop.h
• Move public headers from shared/; some of these expose functions whose implementation is in libshared which is not installed, what to do? Into libweston/include/libweston/ or shared/include/libweston/?

The follwing to be done in further MRs:

• Move GL-renderer into new subdirectory.
• Move each backend into new subdirectories.
• Start separating private and backend bits out of libweston.h to set up new private headers.