Add Super+Scroll actions

[lenemter]

Fixes #815
Fixes #1178
Closes elementary/switchboard-plug-mouse-touchpad#215

[leolost2605]

I really really love this!

[leolost2605]

On second thought I think it might be better to make the void triggered () signal a bool triggered (), return that for propagation and have everything that needs a super scroll action add their own. This would stay more in line with how events, etc. are generally handled and avoids having to add even more api to the windowmanager.
What do you think?

[lenemter]

@leolost2605 we need to implement our own objects hierarchy for this to make sense. I think having one signal and allow anyone to connect to it is simple and shouldn't produce any issues when used correctly.

[leolost2605]

@leolost2605 we need to implement our own objects hierarchy for this to make sense

I'm not sure I understand what you mean here?

[lenemter]

How are we gonna propagate the signal in this case? It's not an event, so we can't use Clutter... TBH I don't understand exactly what you mean in your original comment.

[leolost2605]

I meant something like the last commit in #2182 does.
Feel free to give your opinion :)