While Wayland offers nothing general to help us support `focus` on all
window managers, WM-specific implementations are generally possible.
Sway is a tiling window manager that mimics i3, and has a reasonably
powerful CLI that can help us achieve this.
In addition to supporting `focus` for Sway, this change paves the way
for additional WM-specific Wayland functionality by adding a detection
step to wayland.kak, in a similar fashion to detection.kak.
