Add a libeis backend 🍦

This adds a libeis backend which supports clients sending emulated input events. No public listening socket is exposed, clients are expected to go through the RemoteDesktop portal. In order to restrict the device types available to clients according to what was approved via the portal a separate eis context per portal request is created. The communication with the portal happens through a simple dbus interface where cookies are handed out for each eis context so the portal can inform KWin when the portal session is closed/should end.

Portal side: xdg-desktop-portal-kde!279 (merged)