Split display-related code from the main vdagent code

This is a preliminary step for fixing bugs related to Wayland.

vdagent is using X11 to talk to the display manager. Under Wayland, this means we're talking to the XWayland intermediate layer, which doesn't provide full features, causing several regressions. We will need to use another API than X11 to fix it, but as the X11 calls are used in inner parts of the agent, the separation needs to be done cleanly.

This merge requests separate x11-specific calls and structures from the vdagent code so that we can later modify them by introducing another API.