[Feature Request] Show tiled windows in the dock by exposing an API

**Describe the feature you'd like**

Tiled windows are shown in ALT+TAB to raised all of them together. It would be nice to have the same feature in the dock as well. For example, this is what Windows offers (image from [here](https://www.computerworld.com/article/1632403/make-multitasking-a-snap-on-your-windows-pc.html):

At the moment, I couldn’t find a way for Tiling Shell to influence Dash-to-Dock behavior. This likely requires exposing an API from Tiling Shell that Dash-to-Dock could consume.

**Additional context**
Some ideas to brainstorm with:
- We could consider to expose the API via dbus?
- Tiling Shell could emit signals to which Dash-to-dock connects to?
- Tiling Shell is currently attaching tiling data to windows (if it is tiled and the tile coords and size), Dash-to-dock might already read those?