GtkBuilder: Added api to allow private signal callbacks

GtkBuilder: Added api to allow private signal callbacks

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.