Implement support for keeping shellclients in the multitasking view

[leolost2605]

This provides support for keeping a shellclient (the dock) visible in the multitasking view.

Everything one expects currently works. You can launch new apps, switch workspace by clicking on an app on another workspace etc..

Support was added to allow dragging windows to icon groups in the dock.

Goes with elementary/dock#375

Things todo/think about:

• When multiple instances of an app are open the window overview is shown. That's not really nice as it is right now however I wonder what we should do instead? One option that I came up with: Overlay window overview #2350

Fixes #2071 (at least most of the time, sometimes the user time is a bit outdated but better than main ig)

[leolost2605]

This should be ready for an initial review now together with elementary/dock#375
Lots of testing and feedback would be appreciated :)
Especially testing with different monitor scales and wild monitor transforms like rotations, flips, etc. etc. would be useful since we have to do quite a bunch of coordinates conversion when dropping winows on the dock icon groups

[lenemter]

LGTM! There are two issues:

1. Missing X11 support
2. Window clones have unfocused styles in multitasking view

But we can deal with them later

[leolost2605]

@lenemter I added support for X here, the other thing I'd like to do in a follow up :)