Emitter .apply() calls should use null instead of this

[samreid]

Discovered in phetsims/scenery#928, For unknown reasons Emitter is passing this for the apply calls. I discovered this led to a problem in SimpleDragHandler which was incorrectly using this instead of self.

[samreid]

I fixed the main problem with this, but there are still several other occurrences that will need attention.

[samreid]

I fixed the validation case. The other site that is implicitly using this = Emitter for callbacks are emit1, emit2 and emit3 which are deprecated. So perhaps that's all we need for this issue. @zepumph are you available to double check that this change is reasonable?

[zepumph]

These changes look good.

I also agree that it doesn't make sense to change deprecated methods from something like:

this.activeListenersStack[ lastEntry ][ i ]( arg0, arg1, arg2 );

to

this.activeListenersStack[ lastEntry ][ i ].call( null, arg0, arg1, arg2 );

Hopefully we can update usages soon.