Decode keyboard input as latin-1 whenever appropriate

Decode keyboard input as latin-1 whenever appropriate

* src/keyboard.c (kbd_buffer_get_event_1): Use `coding'
property if present.

* src/termhooks.h (enum event_kind): Document meaning of
`coding' property on text strings.

* src/xterm.c (handle_one_xevent): Set coding property on text
to latin-1 if it was obtained with XLookupString.