[RFC][python] Add support for commands implemented in Python

[Tom Tromey]

>>>>> "Eli" == Eli Zaretskii <eliz@gnu.org> writes:

>> +@defmethod Command complete text word
>> +This method is called by @value{GDBN} when the user attempts @key{TAB}
>> +completion on this command.

Eli> Only TAB, or the other completion-related keys? what about M-?, for
Eli> example?

Yes, it is also used to implement M-? and the CLI "complete" command.

Tom