Introduce "struct ui"

Introduce "struct ui"

This is a step towards supporting multiple consoles/MIs, each on its
own stdio streams / terminal.

See intro comment in top.h.

(I've had trouble picking a name for this object.  I've started out
with "struct console" originally.  But then this is about MI as well,
and there's "interpreter-exec console", which is specifically about
the CLI...

So I changed to "struct terminal", but, then we have a terminal object
that works when the input is not a terminal as well ...

Then I sort of gave up and renamed it to "struct top_level".  But it
then gets horribly confusing when we talk about the "top level
interpreter that's running on the current top level".

In the end, I realized we're already sort of calling this "ui", in
struct ui_out, struct ui_file, and a few coments here and there.)

gdb/ChangeLog:
2016-06-21  Pedro Alves  <palves@redhat.com>

	* event-top.c: Update readline-related comments.
	(input_handler, call_readline): Delete globals.
	(gdb_rl_callback_handler): Call the current UI's input_handler
	method.
	(change_line_handler): Adjust to set current UI's properties
	instead of globals.
	(current_ui_, current_ui): New globals.
	(get_command_line_buffer): Rewrite to refer to the current UI.
	(stdin_event_handler): Adjust to call the call_readline method of
	the current UI.
	(gdb_readline_no_editing_callback): Adjust to call the current UI's
	input_handler method.
	(gdb_setup_readline): Adjust to set current UI's properties
	instead of globals.
	* event-top.h (call_readline, input_handler): Delete declarations.
	* mi/mi-interp.c (mi_interpreter_resume): Adjust to set current
	UI's properties instead of globals.
	* top.c (gdb_readline_wrapper_cleanup): Adjust to set current UI's
	properties instead of globals.
	(gdb_readline_wrapper): Adjust to call and set current UI's
	methods instead of globals.
	* top.h: Include buffer.h and event-loop.h.
	(struct ui): New struct.
	(current_ui): New declaration.