add -s option to ignore scrolling events (#62)

xinput: ignore duplicate XDevicePresenceNotifyEvents

When reacting to a XDevicePresenceNotifyEvent and snooping on all
devices, this can (sometimes?) cause a bunch of duplicate
XDevicePresenceNotifyEvents to show up, which then puts us into an
infinite loop of snooping and reacting to these events.

Store the serial of the event and ignore any duplicates.

Might fix #48.

release 1.6

roll 1.5

call this version 1.4

version 1.3

release version 1.2

version 1.1

init kids to NULL so the later check is correct

prevents XFree trying to free junk in the error path