added "entry" function to GemWindow

for unified output
make devID a signed int

for better compatibility with MPX
(and since "unsigned" can be represented even less by "t_float")
use signed devIDs for multi-pointer
use new event() method
multipointer support for [gemsdlwindow]

seems to do nothing.
do we have to enable it somewhere, or is this a limitation of the used SDL?

LATER: protect the use of "event.*.which" with some #ifdefs