Introduce window grouping

I believe, that it would be amazing to create abstraction client_group with such properties:

1. All arrangement will be done inside group(Mono, Tiled, etc)

2. Group contains 1..N clients. Maybe 0 in case will be needing "persistent" groups

3. Workspace contains 0..K Groups

4. Groups might be able to nest, to introduce more complex layouts:

   For example: Split, where on left there is a Mono group of two windows, and single window on the right

5. Groups should have universal interface for binding Workspaces and Clients