gdb: introduce a per-interpreter event servicing method

This allows an interpreter to override internal calls to
gdb_do_one_event in case the former needs to handle alternate event
sources.

The default action is to call gdb_do_one_event and this is not overriden
in current internal interpreters. However this feature allows to easily
embed Tcl/Tk in insight that needs to concurrently handle Tcl events for
GUI handling.

In all cases, an interpreter event servicing method must call
gdb_do_one_event at some point.

All internal event servicing calls from gdb now direct to the
interpreter-specific method rather than gdb_do_one_event itself.