In preperation for composite objects, for better encapsulation
the following APIs are added to allow handling of signals declared
in the XML with callbacks declared statically.

  o gtk_builder_add_callback_symbol[s]()

    Adds a symbol to the internal symbol hash

  o gtk_builder_lookup_symbol()

    Looks up a symbol, exposed in case added symbols are used
    in conjunction with gtk_builder_connect_signals_full()

The default implementation of gtk_builder_connect_signals() now
does not have a strong requirement on GModule (or a strong requirement
on symbols being declared in the global namespace). Instead GModule
is used as a fallback in the case that symbols are not declared
explicitly.