NotifyingSignal1 is generally useful, hoist it up into flambe.platform.

Also renamed it to HeavySignal1, it might get extra features later.
It sounds like FlashAccelerometer isn't ready yet, disable for now.
Always initialize the orientationUpdate signal.

It's handy to be able to stick a listener on orientationUpdate without
explicitly having to check orientationSupported (touch and keyboard
signals are like this too).
Accelerometer documentation and cleanup.
Update hasListenersValue after an emit.

hasListeners() while dispatching may return the wrong value (disconnects
are deferred until the end of the current dispatch).