feat($rootScope): implement event.stopPropagation() for $broadcast-ed events

[just-boris]

It would more consistent if both ways to trigger an event will have stopPropagation feature.

Implemented as described in #914

Also there is a related question on stackoverlow.

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[just-boris]

I signed it as just-boris@hotmail.com

[just-boris]

I signed it!

[just-boris]

Maybe it should split that complicated expression into more ifs?
But then it will be completely different than traversal in $digest

[googlebot]

CLAs look good, thanks!

[Narretz]

I am not sure what the reasons were for not having stopPropagation on broadcast. However, what's the use case for using stopPropagation this way? Normally, you use events pretty sparingly in an angular app.

[Narretz]

iit -> it

[just-boris]

@Narretz fixed. Now all tests are passing fine.

The reasons are similar to stoping $emit-ed events. If some component on top level already handled this event (showed popover, for example), then children don't need to handle this event again. Now as a workaround I have to use some flags and check that on children and code looks ugly.

I guess that wasn't implemented earlier, because it is more rare case than $emit and harder to implement (you need to stop only one branch in the tree and go on by others).