More accelerometer updates to fit into Flambe

Separated EventGroups so they are independent. Added private NotifyingSignal1, so Accelerometer can stop when it is not in use.

Signed-off-by: Bradley Baysinger <bradley@seven2.com>
Merge branch 'accelerometer' of https://github.com/braddelcat/flambe into accelerometer

Signed-off-by: Bradley Baysinger <bradley@seven2.com>

Conflicts:
	src/flambe/platform/html/HtmlAccelerometer.hx