add window count to focused_window

[thigg]

It would be nice, if the focused_window block could be used as title bar replacement.

The missing bit of information for me is the count of windows on this workspace.
I'd like to display something like:

2|Firefox....

if I have keepass and firefox open on this workspace.

[MaxVerevkin]

It's probably possible with sway_ipc but not wlr_toplevel_management driver, which can report only the total number of windows on all workspaces.

PRs welcome.