Implement gdb.execute_mi

Implement gdb.execute_mi

This adds a new Python function, gdb.execute_mi, that can be used to
invoke an MI command but get the output as a Python object, rather
than a string.  This is done by implementing a new ui_out subclass
that builds a Python object.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=11688
Reviewed-By: Eli Zaretskii <eliz@gnu.org>