Pinned/sticky/show on all workspaces floating windows

[YaLTeR]

Add some way to show a floating window on all workspaces. For things like picture-in-picture, webcam, etc.

I'm not entirely sure what would be the best way to structure the code.

1. UX-wise, it would make sense to have an extra floating space for pinned windows on a Monitor, that renders above all workspaces. This would require somehow integrating keyboard navigation between workspace-local and pinned floating spaces, so e.g. focus-window-up considers windows in both.
2. Other compositors seem to duplicate pinned windows in all workspaces. So they look like they sit inside each workspace's own floating space, but on all workspaces at once. This makes keyboard navigation obvious, but our Layout code really doesn't expect one window to live in several places at once.

I think option 1 should be easier to implement and result in clearer UX, but as usual, need to try to see what kind of roadblocks I bump into.

Of course both of these should have actions to toggle/set pinned status, a window rule to open-pinned, probably a window rule matcher for is-pinned.