gdb/mi: use separate classes for different types of MI command

gdb/mi: use separate classes for different types of MI command

This commit changes the infrastructure in mi-cmds.{c,h} to add new
sub-classes for the different types of MI command.  Instances of these
sub-classes are then created and added into mi_cmd_table.

The existing mi_cmd class becomes the abstract base class, this has an
invoke method and takes care of the suppress notifications handling,
before calling a do_invoke virtual method which is implemented by all
of the sub-classes.

There's currently two different sub-classes, one of pure MI commands,
and a second for MI commands that delegate to CLI commands.

There should be no user visible changes after this commit.