handleKeyPress: Fix shadowing of global 'index' variable
GCC warns:
event.c: In function 'handleDestroyNotify':
event.c:676: warning: declaration of 'index' shadows a global declaration
/usr/include/string.h:309: warning: shadowed declaration is here
event.c: In function 'handleKeyPress':
event.c:1435: warning: declaration of 'index' shadows a global declaration
/usr/include/string.h:309: warning: shadowed declaration is here
So let's rename "index" to "widx".
Note that there is a more serious shadowing in this same function,
event.c:1686: warning: declaration of 'wwin' shadows a previous local
but this is more complicated to solve.