Feat: sway-mode module

[Rodrigodd]

Add a new module, similar to the waybar "sway/module" module. It shows the current mode under sway. For that, it needs to communicate with the Sway IPC. In this PR, the module is called "sway-mode" because "sway/mode" didn't work in my YAML config file (it would probably work if I surrounded it with quotes, but I didn't test).

This PR has two commits: the first one implements the module using a new sway connection. The second one refactors the sway Client, under clients::compositor::sway, to allow dynamically registering events, instead of only handling Workspace update ones. This second commit avoid creating a new socket for each module that needs to communicate with the Sway IPC, not sure if that is a problem to begin with.