Don't apply keyboard decoding to unread-command-events.

Don't apply keyboard decoding to unread-command-events.
* src/keyboard.c: Apply keyboard decoding only to events that come directly
from the tty, not from unread-command-events (bug#14368).
(read_event_from_main_queue): New function, extracted from read_char).
(read_decoded_char): Remove.
(read_decoded_event_from_main_queue): New function to replace it.
(read_char): Use it.
(read_key_sequence): Use read_char rather than read_decoded_char.

Fixes: debbugs:14403