move MIN_DEVIATION constant to __init__.py

By moving the "constant" we will allow it to be modified before it is
used in reg_events module.
abstract axis events and add support for AXIS_MAP_SPACENAVD

Use an abstraction on the default reg events which makes it easier to
dynamically set the motion_* variables. This makes implementing
AXIS_MAP_SPACENAV, for compatability with spacenav programs, easier.
track libspacemouse' change of grab/ungrab to set_grab
update SpaceMouse to use set_grab
update __all__ for set_grab
Add wrapper for _device_get_max_axis_deviation()

And at support to SpaceMouse class for max_axis_deviation as a
property.