[Ada] New GDB/MI command: -ada-tasks-info

[Ada] New GDB/MI command: -ada-tasks-info

This patch introduces a new GDB/MI command: -ada-tasks-info,
which is meant to be the MI equivalent of the CLI `info tasks'
command.

This new command returns an array, with each row corresponding
to one task.  For now, the columns of the array corresponds to
the columns displayed in the CLI output.

gdb/ChangeLog:

        * ada-lang.h (struct inferior): Declare.
        (print_ada_task_info): Add declaration.
        * ada-tasks.c (print_ada_task_info): Make non-static.
        * mi/mi-cmds.c (mi_cmds): Add "ada-task-info".
        * mi/mi-cmds.h (mi_cmd_ada_task_info): Add declaration.
        * mi/mi-main.c: #include "ada-lang.h".
        (mi_cmd_list_features): Add "ada-task-info" to the list
        of supported features.
        (mi_cmd_ada_task_info): New function.