WIP: refactor and humanize helicopter acceleration

Put a smooth function on top of velocity and
angular velocity for increased usability of
the helicopter mode.
stabilitze navigation against anisotropic screens

Signed-off-by: Simon Thum <simon.thum@igd.fraunhofer.de>
take an offset out of heli navigation input

This intends to increase usability by providing a small no-motion
area for the mouse or finger to rest.

This is highly tailored for our scenes and still needs
some knobs for potential other users!

Signed-off-by: Simon Thum <simon.thum@igd.fraunhofer.de>
Merge remote-tracking branch 'upstream/master'

Conflicts:
	src/Viewarea.js -resolved