Add onAction observable for simple snackbars

[jelbourn]

Adds a way to be notified when the action is invoked.

[rolandjitsu]

Is this still being worked on or what's the status? I could make a PR for this, it seems pretty straightforward.

[rolandjitsu]

Made a PR, 1729, but the tests on saucelabs are timing out.

[rolandjitsu]

Closed my PR, messed up some commits. Is anyone working on this at this point or can I give it another try?

[jelbourn]

@josephperrott?

[michaeljota]

If the PR got merge, this should this still open? I think this can be safely closed.

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