tools: fix grab handling in libinput debug-gui

The libinput context's user_data was used for deciding whether to grab the
event device but also to hold the struct window data for the debug-gui. Worked
fine for the initial batch of devices, but any device coming in late would
just use the first field of the struct window to decide whether to grab or
not.

Fixes #122

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>