Accelerometer API improvements via @braddelcat, with some adjustments.

Mainly name fiddling, and accelerationIncludesGravity is a flag on the
emitted acceleration vector instead of a separate signal. Which is a bit
nicer to deal with when writing games that need to work in both cases.

Also renamed Orientation to Attitude as suggested to avoid ambiguity
with flambe.display.Orientation.
Accelerometer → Motion.

The API is really about a bunch of different motion sensors working in
tandem (accelerometer, gyroscope, compass).
FlashAccelerometer needs a lot of work, remove it for now.