Keys got stuck because unicode is 0 upon SDL_KEYUP events, even if the
same key event sets unicode correctly in SDL_KEYDOWN events.

Work around that for the common case (ASCII) using the fact that both SDL
and X11 keysyms were created with ASCII compatibility in mind. So as long
as we type ASCII symbols, we can map things trivially.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>