sbus: Add meta data structures and code generator
    
    These metadata structures hold the information about all the
    details of a DBus interface. They are typically generated from
    the canonical XML form of the DBus interface, although they
    may also be hand crafted.
    
    Add some handy functions for looking up methods, props, signals,
    in the metadata of an interface. Currently lookups are just done
    by looking through an array. If performance becomes an issue (ie:
    very large interfaces) it would be really easy to sort things
    and use bsearch().
    
    Later commits will include some definitions using this metadata
    and related functions.
    
    DBus interfaces are defined here:
    
    http://dbus.freedesktop.org/doc/dbus-specification.html#introspection-format
    
    The introspection data format has become the standard way to represent a
    DBus interface. For many examples see /usr/share/dbus-1/interfaces/ on a
    typical linux machine.
    
    A word about annotations. These are extra flags or values that can be
    assigned to anything. So far, the codegen supports this annotation:
    
    org.freedesktop.DBus.GLib.CSymbol
     - An annotation specified in the specification that tells us what C symbol
       to generate for a given interface or method. By default the codegen will
       build up a symbol name from the DBus name.
    
    It is possible to confuse the code generator into producing invalid
    C code (with strange method names, for example), but the C compiler
    catches such silliness right away.
    
    Add tests testing basic features of the codegen and poking through
    the metadata it creates. Also test the metadata lookup functions.
    
    Generated code is checked in for easy discovery.
    
    An example of the XML interface definitions can be found at:
    
      src/tests/sbus_codegen_tests.xml
    
    And an example of the generated header can be found here:
    
      src/tests/sbus_codegen_tests_generated.h
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>
    
    (cherry picked from commit b699c4d7f85a5404be1d1ee9450331aea869b886)
    
    Conflicts:
        Makefile.am