Avoid consing extra string when processing GTK native input

Avoid consing extra string when processing GTK native input

* src/gtkutil.c (xg_im_context_commit): Use
`decode_string_utf8' to decode input text.

* src/keyboard.c (kbd_buffer_get_event_1): If coding system is
Qt, simply return the string without decoding it.

* src/termhooks.h (enum event_kind): Document meaning of Qt as
coding system in a multibyte keystroke event's string argument.